That's not to discount the impressive job Republicans have done trying to hold Democrats off. In a year that had pretty much everything going against them (24 seats to defend, Donald Trump, an electorate that favors Democrats, Donald Trump), Republicans have managed to keep most of the races on this list competitive — and even knock a few off, like Ohio. In fact, GOP analyst Bruce Mehlman notes that since the first debate, Trump has fallen in every Senate battleground, yet Senate Republicans are up in the polls in 10 of them.

But no matter how perfect a campaign Republicans have tried to run, the reality is it may not be enough to survive a Clinton win by six or more points in these states. Pile on the toss-up factor: Since 1998, Cook Political Report found that at least two-thirds of states ranked toss-ups usually wind up going to one party. And then you get a sense for why Senate Republicans may be pushing the panic button

Here are the top 10 Senate races most likely to flip parties, ranked in order of least (10) to most (1) likely. To the final Senate Friday Line of 2016!

10. Arizona (Republican held): It's getting surprisingly hard to fill this tenth spot, as Sens. Rob Portman (R) and Michael Bennet (D) clean up in Ohio and Colorado, respectively. In Arizona, Sen. John McCain (R) is up over his centrist Democratic challenger, Rep. Ann Kirkpatrick, by an average 14.5 points, according to a RealClearPolitics average of recent polls. But Clinton's campaign is investing time and money to get out the vote in this rising swing state, which will boost Kirkpatrick. The early voting favors Democrats over this time in 2012, too. Democrats have long said the Latino vote will break late, but can they get enough out to vote in big enough numbers to make a difference? (Previous ranking: 10)

9. Florida (R): Since this fall, Sen. Marco Rubio (R) has led or been in a tie with his Democratic challenger, Rep. Patrick Murphy, in all but two public polls. That includes now, when the RealClearPolitics average has Rubio up by 3.5 points. Yes, that is more or less a margin-of-error race. But Murphy is going to need all the help he can get to upset Rubio, and it was decidedly not helpful when Senate Democrats' campaign arm canceled ad reservations for him, preferring to spend the money on other, less-expensive races on this list. (A Senate Democratic super PAC is putting money back in, though.) As with some of those other races, who wins may come down to which presidential candidate wins, and by how much. (Previous ranking: 9)

8. North Carolina (R): North Carolina is still a competitive race, which is partly a reflection of what a major battleground it has become in the presidential race. But the Senate race moves back one slot mostly because other battles — like Missouri — have become more competitive. Also because Sen. Richard Burr (R) is leading Democratic challenger Deborah Ross in four of the past five public polls. At this point, Ross's likeliest route to victory is to hope for a sizable Clinton win, which is possible. (Previous ranking: 7)

7. Indiana (OPEN): Former governor and senator Evan Bayh (D) enjoyed a two-week honeymoon and double-digit leads over Rep. Todd Young (R) when he decided at the last minute this summer to run for the Senate again. But he has spent the rest of the campaign back on his heels thanks to headlines about his lobbying career after leaving the Senate in 2010 and tough questions about how much time he has spent in Indiana since then. Democrats say Bayh's reputation in the state takes precedence over all this. They may be right: A recent local poll found 71 percent of Indiana voters said questions of Bayh's residency did not affect their decision. This state moves back in competitiveness because other states have moved up. (Previous ranking: 4)

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6. Missouri (R): "We should start every day acting like we’re two points behind," Sen. Roy Blunt (R) recently told his supporters. But the freshman senator may have started acting like that too late. His slow-starting campaign is one reason this Senate race in a state that trends red at the federal level is surprisingly competitive. In the past two months we've moved it from 10 to 8 to 6 on our list. The other reason is Blunt's savvy Democratic challenger, Missouri Secretary of State Jason Kander. (Seriously: Watch the Kander ad below.) RealClearPolitics' average of recent polls has Blunt with a scant one point lead. But Missouri is not the most competitive on our list because this is also a state Trump is expected to win. (Previous ranking: 8)

5. Nevada (OPEN): We've gone back and forth this cycle about Rep. Joe Heck's (R) chances to flip retiring Senate Minority Leader Harry Reid's (D) seat red. Final verdict: It's 50/50. Heck had impressively led Reid's hand-picked opponent, former state attorney general Catherine Cortez Masto, in nearly every single public poll. Until The Tape. To the ire of some Trump supporters in the state, Heck rescinded his support for Trump the day after it came out, and he has since privately acknowledged he is in a “very difficult situation” for it. Right now, slight momentum goes to Democrats. (Previous ranking: 4)

4. New Hampshire (R): This race between two well-liked New Hampshire political titans has always been as close as they come. It will come down to whether Sen. Kelly Ayotte (R) can outperform Trump by enough to hang on over Gov. Maggie Hassan (D). (The latest RealClearPolitics average of recent polls has Trump down by 6 points and Ayotte with a 1.3-point lead.) New Hampshire does have a history of ticket splitting, but if Ayotte wins that many ticket-splitters, it would be a historic achievement. (Previous ranking: 5)

3. Pennsylvania (R): This race is our most competitive. Like most Senate Republican candidates on this list, Sen. Pat Toomey (R) has done a remarkable job keeping his race in this swing-state race tight in the Year of Trump. Polls here for the Senate race are swinging wildly in either direction, but the bottom line is Toomey has an ever-so-slight 1.3-point lead over Democrat Katie McGinty. The question for Toomey is: Can an incumbent outperform Trump by five to six points? Like New Hampshire, if he pulled it off, it would be very impressive. (Previous ranking: 3)

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2. Wisconsin (R): Republican-held Wisconsin and Illinois will finish the election cycle on our rankings where they started: Most likely to flip parties. The Republican incumbents are running in states that lean blue in a presidential year. In Wisconsin, Clinton is up by almost seven points, and it's hard to see how Sen. Ron Johnson (R) can beat a well-known opponent who has led him in almost every single public poll but one. Earlier this month, Senate Democrats' campaign arm canceled ads in this state, suggesting they think former senator Russ Feingold (D) won't have trouble closing, which is probably a safe bet. (Previous ranking: 2)