A friend points me to this Paul Mirengoff post on electability versus ideology:
It's also possible to outsmart oneself by selecting a candidate based on ideology. That's because perceived ideological differences among candidates may be more apparent than real. This risk seems particularly pronounced in this year's Republican race. Consider Giuliani and Romney. Giuliani is running as a center-right candidate and Romney as a pure conservative. But as executives of major Northeastern entities, there was little difference between them. Both were moderate to liberal on key social issues, and moderate to conservative on most other matters. Both were pro-choice and both were far from hostile to illegal immigrants in their jurisdiction. Both even supported at least one Democrat during the 1990s - Giuliani backed Governor Cuomo in 1994; Romney voted for Paul Tsongas in the 1992 primary. Now consider McCain and Thompson. In this pairing, McCain seems to be the center-right option and Thompson the conservative. But the two were very closely aligned from 1994-2002 when Thompson held elected office. In fact, Thompson backed McCain's major deviation from conservative principles during this period - the McCain-Feingold campaign reform measure.
Of course, both Romney and Thompson have tacked right in subsequent years, Romney most notably with his pro-life conversion in the fall of 2005. I'm more inclined to believe that there are real ideological differences between the Republican candidates, and that to argue a particular candidate is "electable" based on polls taken in the year before an election is to argue unconvincingly. Any conservative - indeed, any voter - ought to support the candidate who they believe most represents their individual beliefs, and who they further believe will be the most effective leader.