My new Townhall.com column is up, and answers this question by noting that if they do, such conservatives cannot possibly vote for John McCain in the primaries.
Additional background is in the post below,
"McCain Says He'll Vote With Dems On Filibuster Issue," a flashback to Senator McCain's April 14, 2005 interview with Chris Matthews when the Arizona maverick declared for the Dems on the issue of judicial filibusters.