(CNN) With the political conventions behind us and Labor Day approaching, it's the time of year when down-ballot elections start to kick into high gear. At the start of the 2020 cycle, the Senate wasn't expected to be all that exciting, with Republicans largely on defense in red states.
But that's changed. With President Donald Trump trailing in national polls, Democratic challengers raking in millions and demographics shifting across the South, many of those Republican incumbents are sitting in states that don't look as red as they used to.
Democrats need a net gain of three seats to flip the chamber if they also win the White House -- since the vice president would break a tie -- or four seats if Trump wins reelection. Although those net gains are possible, Democrats' path is still complicated by the fact that they're likely to lose a seat in Alabama, where Sen. Doug Jones tops CNN's inaugural ranking of the 10 Senate seats most likely to flip in 2020.
Still, eight of the top 10 seats on this list are held by Republicans. That the GOP is on defense is a reflection of their success in 2014. Half of the senators on this list are Republicans who were first elected six years ago. Two others are longtime incumbents who are facing their toughest challenges yet in South Carolina and Maine.
Besides Alabama, one other Democrat-held seat comes in at the bottom: Gary Peters of Michigan is the only other Democrat running for reelection this year in a state Trump carried, albeit narrowly, in 2016. But Peters is not raising major alarm bells for national Democrats, especially in a state that looks to be moving away from Trump. It's possible this race drops off the list in subsequent rankings.
Related: See CNN's presidential ratings and make your own map here
Several other GOP-held seats could move onto the list in the future. Texas, for example, is a politically evolving state, and the presidential race there is closer than Republicans would like. But this year's Democratic Senate nominee doesn't have anywhere near the kind of money Beto O'Rourke did two years ago, while GOP Sen. John Cornyn isn't as disliked as Sen. Ted Cruz. The Lone Star State is worth watching, though.
So is Georgia. Sen. David Perdue's seat falls in the bottom half of our ranking, but the state's other seat is also competitive. Appointed Sen. Kelly Loeffler faces Republicans and Democrats on the same ballot in the November special election, which will likely go to a January runoff. With so many moving parts, it's too soon to tell whether this seat deserves to be on the list, but there's no question it's causing an extra headache for Republicans.
Two other Republican-held seats -- Kansas and, to a greater extent, Kentucky -- are generating buzz this year, too, but neither is likely to grace the top 10 list anytime soon. Kris Kobach could have jeopardized the open Kansas seat had he won the GOP primary, but national Republicans got the candidate they wanted. And in Kentucky, Democrat Amy McGrath is outraising Senate Majority Leader Mitch McConnell, whose lead has dipped into single digits in a recent public poll. The Bluegrass State elected a Democratic governor in 2019, but McGrath -- who has sometimes stumbled as a candidate -- is still fighting an uphill battle during a presidential year.
Here are the seats most likely to flip:
Incumbent: Democratic Sen. Doug Jones
Jones has been in a perilous position ever since he flipped this seat blue in a December 2017 special election, only narrowly defeating the scandal-plagued Roy Moore. Running in a state Trump carried by nearly 30 points against a Republican who isn't facing sexual assault allegations, there's only so many Republicans Jones can hope to win over. Jones is now facing the Trump-backed Tommy Tuberville, who beat former Attorney General Jeff Sessions in the primary runoff for the right to try to take back Sessions' old seat. Jones isn't running away from his party -- he voted to impeach the President and spoke at the Democratic National Convention -- and any path to victory likely depends on energizing Black voters to turn out. Jones enjoys a healthy financial advantage over Tuberville, and despite the difficulty of the race, national Democrats aren't leaving him out to dry, with one group going up on TV for him earlier this month.
Incumbent: Republican Sen. Cory Gardner
First elected in 2014, Gardner has tried to cut an image of a bipartisan, moderate lawmaker -- touting his sponsorship of the recently passed Great American Outdoors Act in one of his first ads this summer. But even if he were running a flawless campaign, the national environment would be hard to overcome in Colorado, which Trump lost by about 5 points in 2016. Gardner is in a bind: he needs to appeal to voters who disapprove of the President but also not alienate the GOP base, who he needs to turn out in order to win. While former Gov. John Hickenlooper endured a rough Democratic primary and is being attacked for ethics violations, he's a former statewide elected official running on his executive experience, while Democrats try to tie Gardner to the national GOP.
Incumbent: Republican Sen. Martha McSally
Appointed to this open seat after losing the 2018 Senate race, McSally is the only incumbent on this list who hasn't won statewide. The former congresswoman failed to win over suburban women voters in that race two years ago, and she again needs to strike a balance between winning Trump supporters and appealing to the middle. This year, she's running against a former astronaut without a voting record. Democrat Mark Kelly is far outraising her, ending the pre-primary reporting period on July 15 with $21 million in the bank to McSally's $11 million. But the competitiveness of Arizona at the top of the ticket makes McSally slightly less vulnerable than Gardner, given that she could get a boost if Trump does well.
Incumbent: Republican Sen. Thom Tillis
Running in a key presidential battleground, Tillis has to hope things go Trump's way here. The first-term incumbent, who only narrowly won in 2014, ended up avoiding a contentious primary but had to spend money and political capital in the off-year to do it. He didn't make many friends with an infamous flip-flop on Trump's border wall, first penning a Washington Post op-ed against use of an emergency declaration to secure funds for the wall and later, in an appeal to Trump, reversing his position. He's up against former state Sen. Cal Cunningham, a member of the Army Reserves. It'll be an expensive race, with forces on both sides of the aisle already having booked millions of dollars for TV advertising. Cunningham is outpacing the incumbent on fundraising, bringing in $7.4 million to Tillis' $2.6 million in the second quarter.
Incumbent: Republican Sen. Susan Collins
Unlike most of the incumbents on this list who are in their first terms, Collins has the advantage of long-time incumbency. First elected in 1996, the Caribou, Maine, native has relied on an image of a moderate GOP lawmaker. But Democrats' pitch is that she's changed. While her vote to confirm Supreme Court Justice Brett Kavanaugh in 2018 allowed the left to villainize her nationally (which helped raise nearly $4 million for her eventual opponent), it's her vote for the GOP tax plan in 2017 and contributions from the pharmaceutical industry that Democrats are using against her. Republicans, meanwhile, are defending Collins' service to the state and attacking Democrat Sara Gideon, the state House speaker, for things she did in the state legislature.
Maine is one of two states that splits its electoral votes by congressional district, so even though Trump lost the state by 3 points in 2016, he picked up an electoral vote by carrying the sprawling 2nd District, where Collins is from. Trump is unlikely to carry that district by as much this year, but Collins' challenge is encouraging Trump voters there to turn out for her, while not alienating more moderates and independents in the much more populated 1st District.
Incumbent: Republican Sen. Joni Ernst
It's been six years since Ernst ran the "make 'em squeal" ad everyone remembers from 2014. Since then, Trump won the state by nearly 10 points. Ernst spoke at last week's GOP convention, but this year's presidential race looks much closer here, and Ernst -- the first woman elected to federal office in Iowa -- may need to convince voters why she deserves a second term -- even if Trump doesn't. She's up against Democrat Theresa Greenfield, who talks about being a "farm kid" and how Social Security was a lifeline for her family after her first husband, a union electrical worker, was killed on the job. Iowa has elected a Democrat to the Senate less recently than Montana, which is lower on this list, but Democrats have made inroads in the Hawkeye State, flipping two House districts in the 2018 midterms that had backed Trump two years earlier.
Incumbent: Republican Sen. David Perdue
Georgia's political evolution is giving Perdue -- another member of the class of 2014 -- a competitive race. He needs the GOP base to show up for him, but his ads about pre-existing conditions (featuring his sister, a cancer survivor) and drug costs (featuring a Black woman) are signs he knows he needs to win over voters in the state's diversifying and suburbanizing areas, too. Perdue still had a cash advantage at the end of June, but is up against a strong fundraiser in Jon Ossoff, who lost what was the most expensive House race in history in a district outside Atlanta in 2017, and is getting more outside help from national Democrats who've been attacking Perdue's stock trades. But the state's 50% threshold could make this one a challenge for Democrats: If no candidate wins a majority on November 3, the race goes to a January runoff, where turnout typically favors Republicans.
Incumbent: Republican Sen. Steve Daines
Another Republican first elected in 2014, Daines didn't look like he was going to have much of a race until Democrats convinced Gov. Steve Bullock to run for Senate. By the numbers, this looks like a red state -- Trump carried it by more than 20 points in 2016, and that's largely why it falls where it does on this list. But the President is not likely to do as well there this year and there's a recent history of ticket-splitting. Bullock was reelected governor the same year Trump won, and Democratic Sen. Jon Tester won reelection here two years later. Republicans argue that Bullock's presidential primary bid forced him to take a stand against Trump and adopt more liberal positions, so he won't cut as moderate an image as Tester, a working farmer. Democrats, meanwhile, are hammering Daines over his business record in China, trying to make him look like he's not in step with Big Sky Country.
Incumbent: Republican Sen. Lindsey Graham
Unlike some of the other southern states on this list, Trump should win South Carolina without a problem. But it's hard to ignore the money and momentum Democrat Jaime Harrison has picked up for his quest to unseat Senate Judiciary Chairman Lindsey Graham, a Trump skeptic turned close ally. Harrison, a former state party chairman, has been outraising Graham, bringing in about $14 million in the second quarter compared to Graham's $8.4 million. A Quinnipiac Poll from earlier this month showed a tied race. In a state that hasn't elected a Democrat statewide since 1996, Harrison remains a big underdog. But he's giving this three-term incumbent a much closer race than he's used to.
Incumbent: Democratic Sen. Gary Peters
Elected in 2014, Peters is running for reelection in a state that Trump carried two years later. It's debatable how much that should matter, given that Biden is expected to do better in Michigan, and Trump barely won it four years ago anyway. But Republican John James, who lost to Sen. Debbie Stabenow in 2018, has outraised Peters multiple quarters in a row. Republicans have long been high on James as a candidate -- an Iraq war veteran and Black businessman -- even if they recognized his uphill climb against Democratic incumbents in the Wolverine State. But in one sign that national Republicans may see an opportunity here, a GOP-aligned advocacy group recently reserved $4.5 million in the state.