Now, more than ever before, Bush and other candidates find themselves in a corner after the Republican front-runner has fueled new levels of bipartisan scorn over his proposed policy of banning Muslims from entering the United States.
The nominee question is one that Bush no longer answers with a reluctant "yes," asserting instead that a future Trump nominee is simply impossible.
"That's a hypothetical I reject out of hand," Bush said to laughs Tuesday night in New Hampshire when a Democratic voter asked him what he would do if Trump becomes the nominee.
The former Florida governor went on to say that the pundits are "obsessed about" the idea of Trump winning the nomination, but argued that at the end of the day, voters will choose someone who can "actually do the job," who's a "serious" person and someone who "can win."
"And if you measure who you vote for on those three things, I can guarantee you Donald Trump is not going to be the nominee," he added at the town hall in Hooksett. "And I believe I have a fighting chance, a really good chance to make that happen."
On Monday, Bush tweeted that Trump was "unhinged" and at an event earlier Tuesday, Bush alluded to Trump as a "blowhard," condemning his proposal to bar Muslims from entering the United States in the wake of the San Bernardino terror attack.
Bush, whose poll numbers are mired in single digits, faced a round of questions from reporters pressing him on whether Bush's frustrations with Trump could surpass his pledge to support him as a possible nominee.
An agitated Bush refused to envision his bombastic rival as the Republican Party's standard-bearer, saying it would only secure a victory for Clinton.
"I just don't believe that Republicans are going to buy into this language that guarantees that Hillary Clinton has a far better chance of winning," he said.
The nominee question is a difficult one for candidates. Given Trump's popularity among his base, some Republican presidential hopefuls are still being delicate in the hope that they could potentially win over some of Trump's supporters down the road.
Ted Cruz, for example, gave perhaps the mildest response to Trump's Muslim policy, saying he disagrees with him but did not criticize him.
On the other hand, saying on the record that they'd vote for Clinton or the Democratic nominee in a general election could be a political death sentence in the Republican primary.
But it's not just candidates who are grappling with the question. Republican voters are starting to ask themselves what they'll do if Trump becomes the nominee.
Lisa Bowers, a Republican voter from Concord, New Hampshire, said at Bush's town hall that she has agreed with some of Trump's views in the past but his latest comments are "ridiculous and wrong."
"Honestly, if it was between him and Hillary Clinton, I'd vote for Hillary Clinton. At least with that, you kind of know what you're dealing with," she said. "He's just a loose cannon."