Columbus, Ohio(CNN) If Republicans are going to slow Donald Trump's march to the nomination, Tuesday's contests could be their last chance to do so at the ballot box, rather than in a contested convention.
Who's running for president?
Ted Cruz, Donald Trump, John Kasich, Hillary Clinton and Bernie Sanders,
Businessman Donald Trump
announced June 16 at his Trump Tower in New York City that he is seeking the Republican presidential nomination. This ends more than two decades of flirting with the idea of running for the White House.
"So, ladies and gentlemen, I am officially running for president of the United States, and we are going to make our country great again," Trump told the crowd at his announcement.
Sen. Ted Cruz of Texas has made a name for himself in the Senate, solidifying his brand as a conservative firebrand willing to take on the GOP's establishment. He
announced he was seeking the Republican presidential nomination in a speech on March 23.
"These are all of our stories," Cruz told the audience at Liberty University in Virginia. "These are who we are as Americans. And yet for so many Americans, the promise of America seems more and more distant."
Ohio Gov. John Kasich joined the Republican field July 21 as he formally announced his White House bid.
"I am here to ask you for your prayers, for your support ... because I have decided to run for president of the United States," Kasich told his kickoff rally at the Ohio State University.
Hillary Clinton
launched her presidential bid on April 12 through a video message on social media. The former first lady, senator and secretary of state is considered the front-runner among possible Democratic candidates.
"Everyday Americans need a champion, and I want to be that champion -- so you can do more than just get by -- you can get ahead. And stay ahead," she said in her announcement video. "Because when families are strong, America is strong. So I'm hitting the road to earn your vote, because it's your time. And I hope you'll join me on this journey."
Sen. Bernie Sanders, an independent from Vermont who caucuses with Democrats, announced his run in an email to supporters on April 30. He has said the United States needs a "political revolution" of working-class Americans to take back control of the government from billionaires.
"This great nation and its government belong to all of the people and not to a handful of billionaires, their super PACs and their lobbyists," Sanders said at a rally in Vermont on May 26.
And if Bernie Sanders is going to catch Hillary Clinton's Democratic delegate lead, he'll need to pass a big test in Midwestern battleground states.
Your guide to Super Tuesday 3
Voters go to the polls in Florida, Illinois, Missouri, North Carolina and Ohio on Tuesday. Here's what to watch in those contests:
Can Trump keep rolling?
Once again, Trump has dominated the discussion headed into a big election day -- with news cycles in the closing days before the third straight Super Tuesday focused on violence at his campaign rallies. He canceled one in Chicago amid security fears; he said he would look into paying the legal fees of a supporter who is charged with sucker-punching a protestor in North Carolina; he called for protesters who demonstrate at his rallies to be arrested.
Trump has so far managed to avoid any damage from the controversies that have engulfed his campaign from the start. But Tuesday provides another test -- with states like Ohio threatening to chip away at his inevitability as the GOP nominee.
Trump wavers on paying legal fees for violent supporters
It's telling for Republicans who are dead-set on stopping Trump that their hopes Tuesday are resting with two candidates in Florida Sen. Marco Rubio and Ohio Gov. John Kasich who haven't shown much ability to win contests.
That demonstrates the race's evolution: Instead of promoting one of the other candidates, it's all about keeping Trump from amassing the 1,237 delegates he needs to win the party's nomination outright.
But polls show Trump either ahead or within striking distance in all five states -- and a clean sweep could make it next to impossible for any of his rivals, even Cruz, to catch his delegate number by the convention.
If he wins them all, Republicans will be down to a last hope of somehow denying Trump the delegates he needs and driving the race into Cleveland in a contested convention.
Trump could try to hammer that point home at his Mar-A-Lago club in Palm Beach, Florida, on Tuesday night. He'll hold another election-night news conference -- guaranteeing wall-to-wall coverage -- where he could try to drive his foes out of the race.
Do-or-die for Kasich and Rubio
The two most important Republican contests to watch are the winner-take-all states: 99-delegate Florida and 66-delegate Ohio.
Both have home-state favorites in Rubio and Kasich -- and both are fighting to save their campaigns by beating Trump.
Neither has left much doubt: It's win or stay home.
Moments from Marco Rubio's career
Florida Sen. Marco Rubio speaks in October 2013 at the Values Voter Summit, held by the Family Research Council in Washington.
Sen. Mike Lee, left, and Rubio talk before a news conference to introduce their proposal for an overhaul of the tax code in March.
Rubio, center, arrives in the House chamber ahead of Israeli Prime Minister Benjamin Netanyahu's address to a joint meeting of the U.S. Congress in March.
Rubio speaks with the media after delivering remarks during the graduation of small business owners from the Goldman Sachs 10,000 Small Businesses program held in Miami in February.
Rubio talks to aides in December before speaking on President Barack Obama's announcement about revising policies on U.S.-Cuba relations.
Rubio, left, talks to Sen. Lindsey Graham prior to a news conference on Capitol Hill in July.
Rubio talks to reporters in June after a closed door briefing on Capitol Hill.
Rubio speaks to the media in front of a wall dedicated to the victims of the violence in Venezuela as he shows support for the Venezuelan community at a restaurant in Doral, Florida, in April 2014.
Rubio speaks to the media at the Doral restaurant in April 2014.
Rubio checks his phone in March as he arrives for a Capitol Hill news conference to introduce a proposal for an overhaul of the tax code.
Rubio, left, is greeted as he arrives at a restaurant in Doral in February 2014.
Rubio, left, and Florida Gov. Rick Scott speak to the media in Doral in February 2014.
From left, Sens. Cory Booker, Rubio, Kristen Gillibrand and John McCain wait for Obama to deliver the State of the Union address in January 2014.
Rubio addresses an event held by the American Enterprise Institute for Public Policy Research in January 2014.
Paralympian Brad Snyder poses with Rubio during a Team USA Congressional visit in November 2013.
Rubio speaks in November 2013 during a campaign stop for Virginia Attorney General Ken Cuccinelli, the Republican candidate for governor of Virginia.
Rubio confers with McCain as U.S. Ambassador to Syria Robert Ford testifies before the Senate Foreign Relations Committee in October 2013.
Rubio, center, speaks to members of the media as Sen. Ron Johnson, left, and Rep. Ron DeSantis listen during a news conference on Capitol Hill in October 2013.
Rubio returns to the Capitol after meeting with Obama and other Republican leaders about the government shutdown in October 2013.
Rubio speaks to an aide on Capitol Hill as he arrives for the weekly Senate Republican Policy Committee luncheon in September 2013.
What's next if Marco Rubio loses Florida?
Rubio and Kasich have justified their decisions to stay in the race by pointing to their delegate-rich home states as turning points -- even though they haven't racked up many wins on the way.
So far, Rubio has wins in Minnesota, Puerto Rico and the District of Columbia to show for himself. Kasich has strong second-place showings in New Hampshire and Vermont, but no victories.
The situation is especially dire for Rubio, who has trailed Trump by a 2-to-1 margin in recent polls of his home state's likely GOP primary voters.
John Kasich's political career
Ohio Gov. John Kasich speaks at the First in the Nation Republican Leadership Summit on April 18, 2015, in Nashua, New Hampshire. The summit was attended by all the 2016 Republican candidates as well as those eying a run for the nomination. Click through for more on the political career of Kasich:
Kasich speaks during the Republican National Convention at the Tampa Bay Times Forum on August 28, 2012, in Florida.
Kasich, left, and then-Republican presidential candidate Mitt Romney talk with students during a roundtable discussion at Otterbein University on April 27, 2012 in Westerville, Ohio. Romney eventually won the 2012 GOP presidential nomination.
Kasich, left, President Barack Obama, center, and Republican House Speaker John Boehner play the first hole of a golf game on June 18, 2011, at Andrews Air Force Base in Maryland.
Kasich listens to Obama speak during a bipartisan meeting of governors hosted by the President and Vice President Joe Biden in the State Dining Room of the White House on February 28, 2010.
Before officially taking office as governor of Ohio, Kasich talks with reporters after meeting with House and Senate Republican leaders at the U.S. Capitol on December 1, 2010. The GOP leaders talked about ways to create jobs, cut spending and repeal the health care law.
Kasich, right, then a member of the U.S. House, sports "Bush" baseball caps with Texas Gov. George W. Bush on July 14, 1999. The two lawmakers held a news conference at the Ronald Reagan International Trade Center in Washington during Bush's presidential run. Kasich had previously announced that he was withdrawing from the 2000 presidential race and endorsed Bush.
U.S. Rep. Kasich delivers a speech in the Watergate complex in Washington on July 9, 1999, during the College Republican National Committee 53rd Biennial Convention. Other speakers included Republican 2000 presidential hopefuls such as Gary Bauer and Elizabeth Dole.
During a U.S. government shutdown, Kasich, left, and Sen. Pete Domenici, R-New Mexico, speak to reporters outside the White House on January 6, 1996.
Kasich shows a videotape of President Bill Clinton speaking during a news conference on Capitol Hill on December 16, 1995. Kasich was chairman of the House Budget Committee at the time and disputed Clinton's position on the budget.
Kasich, center, shows a thank you note in the form of a check to Speaker of the House Newt Gingrich, left, and Senate Majority Leader Robert Dole (right) on November 17, 1995, in Washington. They were soon engaged in bruising battles with President Bill Clinton over the federal budget.
Backers have been patient with the 44-year-old Florida senator, buoyed by the political talent he has flashed -- especially on the debate stage. And there is a sense that he'd perform better when the race reaches more moderate coastal states. But a poor performance in his home state may mean the end for his 2016 ambitions.
Rubio adviser Alex Conant: Best chance to stop Trump in Ohio is Kasich
Can Sanders win again in the Midwest?
Sanders, the Vermont senator who stunned the political world by upsetting Clinton in Michigan a week ago, still trails in the overall delegate race.
But he's in better shape than Clinton's allies expected him to be at this stage. March 15 was supposed to be the day he was knocked out once and for all. Now, thanks to Michigan, Sanders is seriously competing in three states.
He's betting his criticism of trade deals will hurt Clinton in Illinois, Missouri and Ohio.
Poll: Front-runners lead in Florida, close contests in Ohio
It all has Clinton, he said Sunday, "getting a little bit nervous" about the Democratic presidential race.
"I think she understands that the momentum in this campaign and the energy is with us, and that we have a good chance to win a number of states on Tuesday," Sanders said in an interview with CNN's Jake Tapper on "State of the Union."
Sanders has always said the campaign will shift in his favor starting Wednesday, when the race moves west. A win in any of Tuesday's states would have been a surprise two weeks ago -- but a win in multiple states could give him a critical boost of momentum just as the race moves onto friendlier terrain.
Clinton Southern sweep?
There's a reason Sanders is behind in the delegate count, though: Clinton may be about to run the table in the South.
She's won South Carolina, Texas and everything in between already, and she is expected to finish off the sweep with wins in Florida and North Carolina.
Because Democratic delegates are awarded on a proportional basis, she could win those two, lose the three Midwestern states, and still gain more delegates than Sanders on the night.
Nightcap: Clinton makes her stand in Chicago as Dems battle for Midwest | Sign up
And that's the point Clinton's campaign will make: Even if it's going to be a long slog to the Democratic National Convention in Philadelphia, Clinton is doing everything necessary to win it -- no matter which way something as fleeting as momentum is going in a given week.
If Clinton can defeat Sanders everywhere, though, it would show she's cleared a key hurdle with younger and white voters -- and has done so in swing states that are critical in November.
Cruz's delegate math
Texas Sen. Ted Cruz doesn't lead the polls in any of the five states voting Tuesday, but the contests will test the strength of his campaign's data and organizational capabilities.
There are opportunities for Cruz to rack up delegates -- particularly in down-state Illinois congressional districts, Missouri and North Carolina. His campaign has been carefully calibrating his schedule to seize on those opportunities -- which means he'll try to stay close to Trump in the delegate count even without winning a state.
That's Cruz's first goal on Tuesday, and it leans into the second: drive Rubio and Kasich out of the race.
Ted Cruz allies 'mystified' over super PAC sitting on $10 million
"It will be absolutely clear to everyone that this is a two-man race," Cruz said told reporters Monday in Peoria, Illinois. "I think after tomorrow it will be officially a two-man race. Because no other candidate has any plausible path to 1,237."
Establishment Republicans may never like Cruz. But the violence that has erupted at Trump events in recent days could rally them to Cruz's cause nonetheless, with the hope that the Texas senator wouldn't cause the party long-term damage or embarrassment.
Cruz's argument is about electability. He said on ABC's "This Week" that a Trump nomination would be "a disaster for Republicans, for conservatives. I think it's a disaster for the country because if Donald is the nominee, it makes it much, much more likely that Hillary Clinton wins the general."
CNN's Betsy Klein contributed to this report