The liberal Republicans were particularly vulnerable to being defeated for re-election (Stevens, Coleman (tentatively), Dole, and Smith) or deciding to retire (Martinez, Bond, Domenici, Voinovich, Warner, with Hutchison rumored to be planning her resignation). Domenici and Warner could have stayed in office as long as they wanted, but the other retirees were facing tough races. And, of course, Specter left the party because he knew he would lose otherwise.
Of the liberals on the list, only Grassley, Collins, Snowe, Cochran, and Wicker are likely to be Republican Senators in the next Congress.
Perhaps the NRSC should draw the conclusion that, even if you ignore principle and focus on survival, being a liberal isn’t a good political strategy.