Some might argue that having Hillary Clinton in office would be better for the country and conservatism because she would screw things up so badly that it would actually help conservatives in the long run. But, if people haven't seen through Hillary Clinton after 16 years in the public eye, what makes you think another 4-8 years in the White House would do it? How many Americans saw through FDR? Even as his government policies extended the depression for years after it should have ended, he was voted back into office. Yes, he was a capable war President, but he also did more damage to this country domestically than any other President in history, short-term and long-term, and he's still considered by many people to be one of our greatest Presidents.
But, we don't have to go all the way back to Roosevelt: just think back to 2006. What did we hear then? "We should stay home and teach the Republicans a lesson. They'll take a big beating and it'll be great for conservatism." How did that turn out? From where I am sitting, we have a lot less Republicans in Congress, more squabbling than ever, and we're going to have Republican nominee John McCain. Why? Because sometimes a loss leads to better things, but in politics, as often as not, losing just begets more losing and it can sometimes take a very, very long time for movements to learn from its mistakes. Think back to Roosevelt, whose victories started a 40 year-long dominant cycle for the Democrats and that party's shift to the left in 1972 that started a long slow slide for them that may have finally ended in 2006.
Along those same lines, it's also worth noting that after Barry Goldwater was destroyed in 1964, Richard Nixon, who was even less conservative than McCain, was elected to two terms in the White House. Then, in 1980, Reagan became President. So, there is absolutely no reason to think that if a moderate Republican gets into the White House that it will prevent a conservative Republican from getting in later -- and since I mentioned Reagan, I have heard his name invoked many times in the past few weeks to justify not supporting McCain in the general election.
If Ronald Reagan had been alive and had chosen to endorse a candidate in the primaries, even McCain fans should be honest enough to admit that candidate probably wouldn't have been John McCain. But, McCain's most ardent opponents should also be honest enough to note that Ronald Reagan campaigned for Richard Nixon and Gerald Ford, both of whom were to the left of John McCain. So, were he still alive, Ronald Reagan would almost certainly campaign for McCain against Hillary or Barack and you can be sure that he would not approve of conservatives who say that they'd rather have a liberal Democrat in the White House than a far-from-perfect Republican. So, whether the question is "What would Reagan do" or "what would a principled conservative do" in November, the answer would be the same: vote for John McCain.