Much has been made about whether Republicans will ever cut ties with President Trump over his attempt to hold up military aid to Ukraine to get political dirt on former Vice President Joe Biden. And although there are some signs of the party breaking with Trump, one important reason why we haven’t seen GOP senators break with Trump en masse is because many are up for reelection — in total, there are 23 Republican-held seats up in 2020.

And in five of these races, Republicans face particularly tough reelection battles. Election forecasters have rated these races as the most competitive Senate contests in the 2020 cycle, and in each race, the partisan lean of their states is less than 10 points more Republican than the country as a whole. In fact, in the case of Maine and Colorado, they actually lean slightly Democratic. These senators are especially vulnerable in a general election if Trump’s standing declines, but they’re also in a position where they can’t afford to alienate their party’s base.

Five especially vulnerable GOP seats are up in 2020 Republican Senate seats up for election in 2020 by partisan lean of states Incumbent State candidate filing deadline Race Rating* Partisan Lean Susan Collins ME 3/16/20 Toss-up/Lean R D+4.9 Cory Gardner CO 3/17/20 Toss-up D+1.5 Thom Tillis NC 12/20/19 Toss-up R+5.1 Joni Ernst IA 3/13/20 Lean R R+5.8 Martha McSally AZ 4/6/20 Toss-up R+9.3 David Perdue GA 3/6/20 Likely R R+11.8 OPEN GA TBD** Likely R R+11.8 Dan Sullivan AK 6/1/20 Safe R R+14.9 Cindy Hyde-Smith MS 1/10/20 Safe R R+15.4 John Cornyn TX 12/9/19 Likely R R+16.9 Lindsey Graham SC 3/30/20 Safe R R+17.2 Bill Cassidy LA 7/17/20 Safe R R+17.3 Steve Daines MT 3/9/20 Safe R R+17.7 Mitch McConnell KY 1/10/20 Likely R R+23.3 OPEN KS 6/1/20 Likely R R+23.3 Ben Sasse NE 3/1/20 Safe R R+24.0 Tom Cotton AR 11/11/19 Safe R R+24.4 OPEN TN 4/2/20 Safe R R+28.1 Shelley Moore Capito WV 1/25/20 Safe R R+30.5 Mike Rounds SD 3/31/20 Safe R R+30.6 Jim Inhofe OK 4/10/20 Safe R R+33.9 Jim Risch ID 3/13/20 Safe R R+34.9 OPEN WY 5/29/20 Safe R R+47.4 *Race rating is the median rating from Inside Elections, Sabato’s Crystal Ball and The Cook Political Report. **This is a special election, and the date hasn’t officially been set, so there is no filing deadline yet. FiveThirtyEight’s partisan lean metric is the average difference between how a state votes and how the country votes overall, with 2016 presidential election results weighted at 50 percent, 2012 presidential election results weighted at 25 percent and results from elections for the state legislature weighted at 25 percent. Note that the partisan leans in this article were calculated before the 2018 elections; we haven’t calculated FiveThirtyEight partisan leans that incorporate the midterm results yet. Sources: Daily Kos Elections, Cook Political Report, Inside Elections, Sabato’s Crystal Ball

So a vote on whether to remove Trump from office could really complicate things for them, especially if the current timeline for a vote on impeachment holds. Senate Majority Leader Mitch McConnell anticipates the House will vote by the end of the year, which means the Senate could vote in January or February, before the filing deadlines to mount a bid in most of these races have passed. This means many of these Republicans could find themselves facing a last-minute primary challenger if they voted to remove, making the electoral calculus of casting such a vote even more fraught with peril.

Granted, coming up with any scenario where 20 Republican senators vote to remove Trump from office — the minimum needed if all 47 Democrats voted in favor — is challenging to begin with, but it’s even harder if Sens. Susan Collins, Joni Ernst, Cory Gardner, Martha McSally and Thom Tillis are off the table because they fear a primary challenger.

And the threat of a primary challenger is a real concern. In fact, both McSally and Tillis already have one. Although the other three don’t currently have challengers, Collins could be especially vulnerable if she breaks with Trump — she’s the most moderate Republican in the Senate and has had lukewarm intraparty support in the past, though it improved markedly after she voted to confirm Brett Kavanaugh to the Supreme Court last year. On the other hand, Gardner and Ernst have voted with Trump much more consistently than Collins, but a pro-removal vote would likely still tarnish them in the eyes of the party’s base.

To be sure, we don’t know how the impeachment saga will unfold, but the electoral ramifications on the GOP side are something to keep in mind. That’s because even if there is some movement in a pro-impeachment direction among Republican voters, there may still be some senators up in 2020 who fear the potential dangers of angering the party base and getting a serious primary challenge.

We’re tracking impeachment polls; check them out!