In general, nobody likes a spoiler. But John O'Sullivan argues that a conservative refusal to endorse John McCain may be rational even if one stipulates that Hillary Clinton or Barack Obama would be substantially more liberal:
Many conservatives believe that the key question in this election is: Are there to be two multiculturalist open-borders parties or one? If McCain’s election were to make the GOP fundamentally similar to the Democrats on immigration, bilingualism, racial preferences, and all the National Question issues, that would be a resounding historical defeat for conservatives.
The willingness of a President McCain to cooperate with the Democrats would give such issues as an immigration amnesty a better chance of passage than under a President Hillary or Obama even against strong GOP resistance in Congress. Opponents of such policies, despite enjoying majority support among the voters, would find themselves politically marginalized. On the other hand, a united Republican opposition might well stop a Democratic White House from passing these measures because its party would be nervous of finding itself on the wrong side of a popular issue in the next midterm elections.
I think it's probably true that, in practice, a comprehensive immigration reform is more likely to come in a McCain administration than it would in an Obama or a Clinton administration. So in a narrow sense, O'Sullivan's making sense. What's more, a deliberate effort by the nativist wing of the conservative movement to spike McCain would help ensure that, in the future, conservatives whose main priorities aren't immigration kowtow to the restrictionist side of the "National Question." Thus, in a strict sense O'Sullivan is making some sense here.
But this analysis seems to entirely lack context. If electing a pro-amnesty Republican whose administration fails to ban affirmative action programs would be the end of the conservative movement, then Ronald Reagan's eight years in office were, just like George W. Bush's, a "resounding historical defeat for conservatives." Conservatives can be purists if they like, but the reality is that these are issues on which people who agree with O'Sullivan have never held the whip-hand, and it's unlikely that they ever really will as long as the GOP remains the party of business first and foremost.