Donald Trump looks increasingly likely to be the Republican nominee for the White House. As he gains momentum, Trump has won a series of endorsements from high-profile Republicans. Former presidential rivals Chris Christie and Ben Carson made the calculation that it’s better to be with Trump than against him, and so have a growing number of Republican members of Congress.

For many Republican lawmakers facing re-election in competitive moderate or swing districts, however, the prospect of Trump at the top of the ticket must be absolute agony.

Take Carlos Curbelo, a Florida Republican up for re-election in 2016 in a battleground congressional district. Curbelo is one of the most vulnerable members of the House GOP, so  how is he handling Trump? He has spoken out against the Republican front-runner in the past, calling him “crass and insulting and offensive” and vowing not to support him if he wins the nomination. On Thursday, CBS Miami suggested that Curbelo might be willing to take his anti-Trump crusade a step further. “Republican Curbelo Open to Voting for Clinton,” its headline declared, as it reported that the lawmaker “has been telling some Democrats in his district he would vote for Hillary Clinton over Donald Trump if those were his two choices, according to those who have spoken to him.”

Complicating matters, Curbelo took to Facebook to repudiate the allegation. “Dear friends, just to dispel a rumor started by a misleading news story, I have no intention of supporting either of the candidates currently leading in the delegate count of each respective party,” Curbelo wrote. (A spokesperson for Curbelo declined further comment, pointing to the Facebook post instead.)

Meanwhile, Kim Romano, a source cited in the original media report who claimed that Curbelo told her he would vote for Clinton over Trump, appears to be standing by version of events. “I really thought he meant it,” Romano told CBS Miami on Thursday, adding: “I guess he told me what I wanted to hear.”

The back-and-forth provides a window onto the difficult situation that vulnerable Republicans find themselves in as Trump leads the GOP presidential field. It doesn’t make much sense for Republican lawmakers to go so far in opposing Trump as to proclaim support for Clinton, since that would inevitably open them to attack from members of their own party. But moderate Republicans up for re-election who fail to distance themselves from Trump risk being tied to the candidate and statements he has made that have sparked outrage and offense.

Whatever happened, the incident highlights the pressure faced by any Republican uncomfortable with the prospect of a Trump presidency to firmly pick a side as the general election draws near. Plenty of conservatives don’t like Trump. But presidential contests are a binary affair. Voters are asked to pick either the Republican or the Democratic in the race, and any elected official who declines to align themselves with either candidate risks backlash and confusion from voters who may not understand what they stand for if they reject the standard-bearer for their party.

Centrist lawmakers typically take the months leading up to a re-election battle to carve out an inoffensive, middle-of-the-road path, working to establish a record that will be palatable to swing voters. Trump, a candidate who peppers his speeches with insults that might be deemed out of bounds in any other election cycle and who has taken an axe to sacred cows of conservatism, makes that task significantly more challenging for vulnerable Republicans.

Of course, that’s probably not something that moderate conservatives want to admit publicly. So what’s a vulnerable Republican to do?

Curbelo is far from the only vulnerable Republican faced with a tough choice. There’s growing speculation that a Trump nomination would put the House Republican majority at risk, an idea that would have been deemed inconceivable before he appeared on the presidential primary scene.  For now, at least some Republicans seem to be doing their best to defer any difficult decisions. “Vulnerable House Republicans are adopting a strategy for campaigning with Donald Trump as their nominee: Bury their heads in the sand and hope for the best,” Politico reported earlier this month, adding: “Their plan, in a word, is to ignore him.”

If Trump secures the Republican nomination, he’ll be a lot harder to ignore. Vulnerable Republicans who have so far attempted to stake out a middle ground, may have to pick a side once and for all.