(CNN) It's been a rough couple of weeks for President Donald Trump, and that's squeezing some GOP senators who were already facing competitive reelections.
Republicans were optimistic that a Supreme Court vacancy would galvanize their supporters -- and with confirmation hearings for nominee Amy Coney Barrett beginning this week, they may still see that boost. But the hearing could also give Democrats a chance to prosecute their arguments on health care, a winning issue for them in the 2018 midterm elections. Democratic nominee Joe Biden led Trump 53% to 42% in CNN's poll of polls as of Sunday. And three weeks from Election Day, with millions of voters already casting their ballots, the national environment seems to be benefiting Democrats running for Senate -- even if one of their candidates in a top-tier race is facing negative headlines about his personal life.
The first presidential debate, during which Trump refused to condemn White supremacy, didn't help down-ballot Republicans. And his diagnosis with Covid-19, followed by that of three GOP senators, may have only cemented the pandemic -- and Trump's poor handling of it -- in voters' minds, making it more difficult for Republicans to pivot to the economic message that they think is their best play for 2020.
Interactive: 2020 Senate race ratings
Blue states like Colorado and Maine now look even tougher for Republicans. Despite Sen. Susan Collins being a longtime incumbent, who's arguably a better candidate than first-term Republican Sen. Thom Tillis, Maine now moves ahead of North Carolina on this list of seats most likely to flip partisan control.
The rankings are based on CNN's reporting, as well as polling, fundraising and advertising spending data.
South Carolina has been on this top 10 list since the end of August. Democrat Jaime Harrison announced over the weekend that he had raised $57 million in the third quarter, shattering the previous quarterly record for a Senate candidate set by Beto O'Rourke in 2018. The fundraising disparity is a big problem for GOP Sen. Lindsey Graham, who has his moment in the spotlight during the week's confirmation hearings.
But it's not just Harrison who's raising impressive sums. Democrats in other traditionally red states also raked in the cash from July through September. Fundraising reports due on Thursday will give a better sense of the disparities between Democratic challengers and GOP incumbents.
The senator most likely to lose is still a Democrat -- Doug Jones of Alabama -- while Democratic Sen. Gary Peters still faces a competitive race in Michigan. But eight of the top 10 seats most likely to flip are still held by Republicans, who are defending 23 seats this year to Democrats' 12. Democrats need a net gain of four seats to flip the chamber -- or three if they win the White House, since the vice president breaks ties in the Senate. Inside Elections with Nathan L. Gonzales, a CNN contributor, projects Democrats will gain three to five seats.
Interactive: 2020 House race ratings
Democrats' strategy for winning those seats looks similar to their 2018 House strategy: Make sure there are challengers in red states who can raise money and take advantage of a favorable environment. More so than a few weeks ago, Kansas, Alaska and Texas look competitive, in part, because of Democrats' fundraising, although none make this list. Both parties agree that Kansas, where Republican Roger Marshall has struggled to raise money, merits an honorable mention on the list since it's attracting serious spending. Including advertising spending to date and future reservations from candidates and outside groups, Democrats are spending nearly $26 million to Republicans' $30 million in the Sunflower State, according to a CNN analysis of CMAG data as of Monday.
Another race that's making Republicans nervous is the second US Senate seat in Georgia, a demographically changing state. Appointed Sen. Kelly Loeffler is on the ballot with candidates of all parties, including a Republican congressman who's pushing her to the right. A Quinnipiac University poll from late September showed Democrats starting to coalesce around Raphael Warnock, who led with 31%, followed by Loeffler and GOP Rep. Doug Collins in the lows 20s. However, given the unpredictability of a likely runoff -- and who makes it to the top two slots -- it's too early to know whether this seat will flip.
Here are the seats most likely to flip:
Incumbent: Democratic Sen. Doug Jones
No matter how good national polling looks for Biden, it's not likely to get Jones across the finish line in such a Republican state. Tommy Tuberville's ads check all boxes of Republican messaging: He promises to help Trump "drain the swamp," while sandwiching Jones between Biden and House Speaker Nancy Pelosi. Most importantly, though, he's not facing allegations of sexual assault, which should be enough for a Republican to win here after Jones only narrowly defeated the scandal-plagued Roy Moore in a 2017 special election.
Incumbent: Republican Sen. Cory Gardner
There just isn't much of a path for Gardner to hang on in a state that's likely to soundly reject the President in November. In an effort to court ticket-splitting voters, a recent Gardner spot touts the bipartisan split in Colorado's Senate delegation. "You choose one Republican and one Democrat, and it works," Gardner says as a picture of Democratic Sen. Michael Bennet flashes onscreen. Bennet, however, doesn't think it works -- he quickly reiterated his support for Democratic former Gov. John Hickenlooper, who has announced raising nearly $23 million in the third quarter. And while Gardner's ads don't feature Trump, he hasn't made the same commitment as Collins -- for example, to oppose the President's Supreme Court nominee before the election -- because any path to victory requires GOP base voters showing up for him.
Incumbent: Republican Sen. Martha McSally
The only thing that gives McSally a slightly better shot of holding on than Gardner is that Arizona could be more competitive at the presidential level, although public polling earlier this month gave Biden an edge here. Trump carrying the state could potentially boost the former congresswoman, who was appointed to this seat after losing the 2018 Senate race. But she's still in a bind when it comes to shoring up conservative support and appealing to suburban moderates, as a recent debate showed when she would not say whether she's proud of her support for Trump. She's up against a well-funded Democrat in NASA astronaut Mark Kelly, the husband of former Rep. Gabrielle Giffords.
Incumbent: Republican Sen. Susan Collins
Collins moves up a spot on this list, meaning she's now more likely to lose than she was two weeks ago. The Supreme Court fight has nationalized the race and put fresh scrutiny on Collins' 2018 vote for Justice Brett Kavanaugh, which Democrats are now more aggressively deploying in messaging against her. But the four-term senator's biggest problem could be ranked-choice voting, which kicks in if no candidate receives a majority. With recent polling showing Collins under the 50% threshold, it's difficult to envision her winning outright. And if she doesn't, she'll have to contend with the supporters of third-party candidates potentially throwing their support to Democratic state House Speaker Sara Gideon. Inside Elections shifted the race from Toss-up to Tilt Democratic at the beginning of the month.
Incumbent: Republican Sen. Thom Tillis
Shortly after being sidelined by a Covid-19 diagnosis, Tillis may have caught a break, although it remains to be seen how Democrat Cal Cunningham's sex scandal could shake up a race that Republicans acknowledge very much needed shaking up. The Army reservist and Democratic former state senator raised more than $28 million in the third quarter and had been consistently outraising Tillis and leading in polling. Republicans argue that Cunningham's alleged extramarital affair undermines the campaign pitch he's made to be a man of character. Both Tillis and GOP outside groups were quick to seize on the issue in their ads. But there are not enough new data to know how the latest headlines will affect the race, with Democrats skeptical that it'll move many voters, especially as hundreds of thousands of them have already cast their ballots. At this point, Tillis moving down on this list is more a reflection of Collins' worsening fortunes in Maine than any real brightening of his own prospects.
Incumbent: Republican Sen. Joni Ernst
With the presidential race close in Iowa, the Senate race remains a Toss-up. Democrat Theresa Greenfield enjoyed a slight lead, 50% to 45%, over Ernst among likely voters in an early October Quinnipiac University poll. Ernst is still having to apologize for expressing skepticism about the coronavirus death toll, which isn't what Republicans want to be talking about. She's running an ad featuring the state's senior senator, Chuck Grassley. Meanwhile, Greenfield is touting support from Republicans and small businesses owners. Having raised nearly $29 million in the third quarter -- a staggering sum that's dwarfed only by this quarter's other unusually high and record-breaking hauls -- Greenfield should have plenty of resources to carry that message through Election Day. But this is one of the places where Republicans are hoping a Supreme Court battle energizes conservatives and tilts the state back their way.
Incumbent: Republican Sen. Steve Daines
Daines faces better odds than some of his Republican colleagues who are running in states where Biden is ahead. Trump isn't likely to carry Big Sky Country by the same 21-point margin he did in 2016, but he'll likely still win here. That's not necessarily a deal-breaker for two-term Gov. Steve Bullock, though, who's proved he can win statewide at the same time Trump carries the state. Democrats are leaning into their health care message here, with Bullock talking about his own son's heart stopping in a recent ad about protecting coverage for preexisting conditions. Donning his own barn coat, Daines is on air trying to tie Bullock to national Democrats on gun control and taxes, claiming the Democrat is "backed by the liberal mob."
Incumbent: Republican Sen. David Perdue
Republicans are worried about how the diversifying and well-educated suburbs outside of Atlanta could imperil their hold on the state. The fate of first-term Sen. Perdue may have a lot to do with the top of the ticket. A Biden win here would boost Democrat Jon Ossoff, who lost an expensive House special election in 2017. But while Georgia, based on demographics and top-of-the-ticket competitiveness, looks like a harder state for Republicans to hold, Perdue's saving grace may be a runoff. If neither candidate receives a majority of the vote in November, they'll advance to a January contest, where Ossoff wouldn't have the same presidential momentum behind him.
Incumbent: Republican Sen. Lindsey Graham
No one's doubting whether this is a race anymore, with public polling consistently showing the candidates tied. Democrat Jaime Harrison's eye-popping $57 million haul in the third quarter will give him plenty of resources to communicate and help turn out the vote in the final stretch. He's now getting outside assistance from national Democratic groups too. Graham has publicly pleaded for financial assistance, but investment from outside GOP groups isn't likely to level the spending playing field, especially when candidates get more favorable TV rates than outside groups. Republicans are hoping that this week's Supreme Court confirmation hearings will drive conservatives to Graham, who needs to coalesce support on the right. Democrats are trying to exploit that gap with a recent ad from Senate Majority PAC that features a golfer complaining that Graham hasn't taken advantage of his access to the President. A new 60-second Harrison spot, narrated by Viola Davis, features two White voters exchanging worried glances and then subtle smiles as one woman plants a Harrison sign in her front yard, while her neighbor waters his lawn. The implicit message? It's OK to vote for a Democrat in South Carolina. A third-party candidate dropped out of the race and endorsed Graham, but his name is still on the ballot, which means he could take some votes from the senator that Graham can't afford to lose.
Incumbent: Democratic Sen. Gary Peters
Michigan may be moving away from Trump at the presidential level, but the Senate race is still close. A New York Times/Siena College poll released Monday put Peters at 43% to Republican John James' 42% among likely voters, which is much tighter than a June survey from the same pollsters that had Peters 10 points ahead. And Republicans argue that Democrats wouldn't be spending here if Peters were in better shape. Including advertising spending to date and future reservations from candidates and outside groups, Democrats are spending about $55 million compared with Republicans' $43.5 million, according to a CNN analysis of CMAG data as of Monday. It's the rare race where the Republican candidate hasn't been lagging in fundraising. Peters and James both announced raising $14 million in the third quarter. Recent Republican advertising ties Peters to Sen. Bernie Sanders of Vermont and Rep. Alexandria Ocasio-Cortez of New York, but if Biden does well here, it becomes harder for James to pull off an upset, especially if Republicans don't see the kind of tightening at the top of ticket that they'd hoped for at the beginning of the fall.
This story has been updated to include additional detail about the CNN rankings.