What would the conservative pundits do?
Most, I think, would choose to go into exile, withdrawing their support, at least temporarily, from the Republican Party. And that would be an event every bit as significant as the split in the Republican electorate that a Trump candidacy would reflect and intensify. It would mark nothing less than the crack-up of the conservative movement as it's been constituted for the past 35 years.
The Wall Street Journal's influential editorial page and its leading columnists, for example, are ideologically committed to open immigration and an idealistic foreign policy. That would make it impossible for most of them to go along with a Trump candidacy.
Something similar could be said about the literal and figurative descendants of the original neoconservatives at The Weekly Standard and Commentary. These magazines and their most prominent regular contributors are best described as hard-power Wilsonians. They truly believe that American hegemony and military involvement across the globe is nearly always good for the United States, good for Israel, and good for the world as a whole because of the strength and worth of America's universalistic ideals. Trump's xenophobic vision of the country as an armed camp ringed by walls designed to keep out Mexicans and Muslims is inimical to their idealistic vision of the country and its mission.
The same is true of Fred Hiatt's Washington Post editorial page. It holds, too, for Post columnists Charles Krauthammer and George F. Will. Though the latter is a harsh critic of the neocons, he's made his intense dislike of Donald Trump clear from the beginning of his campaign. It's unthinkable that Will would end up supporting his candidacy.
The real question to ask of those opinion journalists in the neocon orbit is which of them would be willing to publicly endorse a Hillary Clinton candidacy against Trump. It's hard to imagine Commentary's John Podhoretz supporting any Democrat for president, but I can certainly imagine him using the pages of his magazine to denounce Trump all the way through the fall. Max Boot has made clear at numerous points in the past that he advises and supports Republicans primarily because of foreign policy; if the Clinton camp asked him post-convention to climb on board to help her defeat Trump, I wouldn't be at all surprised if he took her up on the offer.
William Kristol is trickier. Ever on the lookout for a populist figure to whom he can attach himself and in whose ear he could whisper in the Oval Office, Kristol was at first guardedly optimistic about Trump's candidacy. That abruptly ended when The Donald lit into John McCain for becoming a POW during the Vietnam War. Since then Kristol has repeatedly attacked Trump and repeatedly predicted the certain and imminent demise of his campaign.
When it comes to the National Review, still movement conservatism's flagship magazine, its most prominent staffers have already hit Trump very hard. NR editor Rich Lowry and Trump attacked each other publicly over whether Carly Fiorina castrated him in the September GOP primary debate. Back in late summer, Jonah Goldberg savaged Trump and his supporters (whom he memorably dubbed the "Trumpen Proletariat") for their unconservative ways. And Ramesh Ponnuru has made a point of dismissing the candidate as a nuisance and his campaign as boring.