Justice Anthony Kennedy’s retirement just made the midterm elections even more contentious than they already were.

His replacement will have to be approved by the Senate, and the confirmation process will likely take place in the weeks leading up to the November elections. (Sen. Majority Leader Mitch McConnell has already promised the vote will take place “this fall.”) That means a number of senate Democrats holding endangered seats in Trump-voting states will have to make a difficult decision: support their party leadership and vote against President Trump’s eventual nominee, risking the wrath of voters at home, or vote for the president’s nominee and endanger whatever slim chance the Democrats might have of blocking Trump’s nominee. It’s a particularly perilous decision for moderate Democrats, who are already something of an endangered species in 2018.

There are a handful of these moderate Democrats to keep an eye on. When the president’s first Supreme Court nominee, Neil Gorsuch, was confirmed by the Senate in April 2017, three Democrats crossed party lines to vote for him: Joe Manchin of West Virginia, Heidi Heitkamp of North Dakota and Joe Donnelly of Indiana. Trump won in each of their states, and each senator is up for re-election in the fall in a race rated as a toss-up by The Cook Political Report. There are two other Democratic senators who face similar toss-up races in 2018 but voted against Gorsuch’s nomination: Bill Nelson of Florida and Claire McCaskill of Missouri. Nelson was thought to be worried about drawing a primary challenge from the left if he voted for Gorsuch, and McCaskill reportedly agonized over her decision before casting a vote against Gorsuch. In the year since the Gorsuch vote, the Senate has added one more Democratic moderate whose next election is looming on the horizon: Doug Jones of Alabama, who faces re-election in 2020.

Moderate Democratic senators are faced with a vote that has long-term and far-reaching consequences. The newest justice will do more to reshape the court than Gorsuch’s confirmation did. Gorsuch was a conservative who replaced a conservative. Kennedy is a moderate, and has for years been the swing vote on the court. (He was the tiebreaking vote in cases that legalized gay marriage, put George W. Bush into office, and struck down part of the Voting Rights Act.) Trump is likely to nominate someone more conservative than Kennedy, and now Chief Justice John Roberts, a lustier conservative than Kennedy, will likely be the new swing vote, a striking ideological shift for the institution.

The high stakes of nominations to the high court aside, politicians also want to get re-elected, which is why moderate Democrats have so far remained noncommittal on a future nominee. On Wednesday, after Kennedy’s resignation was announced, Donnelly gave an emphatically milquetoast statement in response to the news of an eventual confirmation vote, a sure sign of how carefully he’ll have to navigate the process; Manchin appeared to voice support for McConnell’s confirmation schedule; and Heitkamp said that “two wrongs don’t make a right,” an allusion to Obama’s scuppered nomination of Merrick Garland.

In recent Trump cabinet nomination votes, the Senate’s Democratic moderates have seemed willing to go along with the president’s choices, perhaps giving some indication as to how they might approach Trump’s Supreme Court nominee. All the endangered Democrats — Donnelly, Manchin, McCaskill, Heitkamp, Nelson and Jones — voted to confirm Secretary of State Mike Pompeo. All but McCaskill and Jones voted to confirm Gina Haspel to be CIA director. The Democrats’ chances of holding back a nominee depend on their ability to reach a majority, which requires securing not only the votes of every member of their 49-person caucus, but also the vote of at least one Republican.

And therein lies the rub: Which Republican, in the twitchy partisan atmosphere of 2018, would cross party lines so boldly, severely damaging their re-election prospects in the process? If there are none to be found, moderate Democrats might well hold their noses and vote for Trump’s nominee.