Neo-conservatives were liberals, once (much as Saruman described orcs: "They were elves, once"). Might they become Democrats again? With Donald Trump presaging a potential outright crackup in the Republican Party, I think it's a very realistic prospect. Trump is, to say the least, not the neo-conservative candidate of choice, but the threat he poses to neoconservatives runs deeper than most. To see why, it's worth digging into exactly how the neo-conservatives originally switched sides.
The origin story of modern neo-conservativism is that some liberals, in the 1970s, grew frustrated with what they took as their cohorts indulging in new age argle-bargle that made them soft in pursuing their own values. Internationally, they perceived concerns about "imperialism" or military overreach as preventing America from being a force for good in the world. They were hawks, but hawks justified by the justness of the American cause. Domestically, they had often supported the civil rights movement in the 1960s but saw little difference between the "old" Jim Crow racial politics and "new" left identity politics of groups like the Black Panthers. They styled themselves as hard-headed social scientists who believed in equality but measured it via data. There's a reason neo-conservatives view themselves as their side's intellectual wing (There are excellent reasons to contest these self-assessments, I'm just going with how the neo-cons viewed themselves).
From this vantage, Trump is conservatism out of the neo-cons' worst nightmare. Domestically, he primarily plays on the social fears of the white base in ways that do not even allow the facade of egalitarianism . His foreign policy is not internationalist in orientation at all and seems entirely uninterested in even pretending to project American goodness abroad. He's openly racist, openly anti-intellectual, and openly fear-mongering.
And against him will almost certainly be ... Hillary Clinton. Someone who absolutely seems to have hawkish tendencies. Someone who seems to surround herself with smart people with good intellectual pedigrees. Someone whose New Democrat husband at least gained the respect, if not the support, of neo-cons in the 90s (they were never among the true rabid Clinton-haters). Someone whose social liberalism is hardly a deal-breaker (neo-conservatives tended to be far more socially moderate than the rest of the GOP).
The GOP that Trump has built has no real space for neoconservatives. But they could easily find a home as the moderate wing of a Clinton-led Democratic Party. If there is any faction within the GOP that I predict to defect not just en masse, but permanently, over to the Democrats following a Trump nomination, it's them.*
* Disclaimer: This post in no way should be read as suggesting that Democrats should alter their position to be more amenable to neo-conservative views. It is pure prognostication; no normative or prescriptive suggestions should be inferred.