The big question hanging over the Republican Party right now is whether or not the party fractures when faced with a Trump nomination. Romney attacked Trump, and his rivals have been similarly harsh. Yet, they all say they'll vote for Trump if he is the nominee because the alternative is Hillary.
This makes sense. Lay off.
There are two reasons that Romney and the rest are attacking Trump.
1) They think he is a weak general election candidate against Hillary. The best case for this is the likelihood that non-white turnout skyrockets with Trump on the general election ballot. Since the Republican Party relies almost exclusively on white voters, that's a problem.
2) They don't trust him on policy. In 2017, if Trump is president, there will be a Republican Congress. That Congress will give him bills to sign that cut taxes. Trump will sign those bills. The problem is, what happens in January, 2019. If Trump is president during the 2018 midterm election, there could easily be a Democratic wave that turns Congress against him. What will Trump do then? Nobody knows. Instead of just signing the bills handed to him by Speaker Paul Ryan, he'll have to deal with Speaker Nancy Pelosi, who was probably the most brutally effective Speaker in modern history. Trump has no real ideological principles, and hence no reason to fight Pelosi the way Rubio, Kasich or Cruz would. That could mean liberal policy outcomes.
So, the reasons Republican elites don't want Trump are electability and lack of trustworthiness on policy. Neither are a reason to vote for Hillary against him. Voting for Hillary because Trump will lose to Hillary is self-defeating, and voting for Hillary over Trump because you don't trust the latter is taking certain policy loss over possible policy loss. Why, then, should Republicans promise to vote for Hillary over Trump in the general election? They shouldn't, given their reasons.
So, let's talk about what Democrats want to talk about here. Trump was endorsed by David Duke, the former leader of the KKK. Trump's Democratic critics see this as prima facie evidence of Trump's racism. Such voters were never going to vote for a Republican anyway, though, and the premise of modern conservatism is that racism doesn't exist anymore, except in a few fringes that should be ignored. To a Republican, here's how the Duke endorsement looks: like a meaningless statement by a fringe figure who should be ignored.
Imagine the Rush Limbaugh caricature of the "feminazi." This cartoon character is a woman so radical that she wants to create a new power structure that is the inverse of the pre-suffrage era, while turning to in vitro fertilization for procreation so that men aren't even necessary for that.
Now, imagine such a hypothetical person endorsing Hillary on the grounds that men cannot be allowed anywhere near the presidency. Should liberals promise to vote for a Republican over Hillary because of this fictitious "feminazi's" endorsement? No. They discount it as some fringe wacko who should be disregarded because Hillary doesn't really resemble the Limbaugh caricature.
Conservatives wouldn't see it that way. And just like liberals in this hypothetical scenario, they don't see David Duke's endorsement as a reason not to support Trump over Hillary. They see his failure to condemn Duke fast enough as an electoral problem, which doesn't mean they should vote for Hillary.
It comes down to this: is racism a real, prominent, pervasive feature of American politics, or has it been successfully banished to the fringes of internet comment threads? If the former, then Trump is part of the problem and one should vote for Hillary. If the latter, then one can disregard Duke's endorsement of Trump and vote on the simple basis of this: tax cuts, yes or no?