Sen. Arlen Specter's defection to the Democratic Party this week is no reason for Republicans to cheer. But his reason for leaving -- he faced an unwinnable primary election next year -- is no cause for soul searching. There is a question Republicans do need to ask: What is it that binds our party together?

In the wake of two successive electoral defeats and the likelihood of a 60-vote Democrat majority in the Senate, what does it even mean to be a Republican today? Moderate Republicans are right to remind conservatives that...