The best way to win the fight for the "moderate" swing voter is to have the best and strongest message. If that message is socialism, then we've lost anyway. Having both primary candidates for president espousing socialism to get the middle... means there is no victory to be had.
Why is this so hard for our GOP "leadership" to understand?
Liberals WILL maintain party loyalty as you say. They DO NOT necessarily back the most liberal candidate -- if they did, Dennis Kucinich would have been the nominee in 2004. Bill Clinton was certainly not the most liberal nominee in 1992; Carter was not the most liberal nominee in 1976.
Parties generally nominate the candidate from within their ranks that has the best chance to win. The idea that some ideology is the overriding criterion for the primaries is ludicrous. Think GHWB was the most conservative in 1988? Dole in 1996? Bush in 2000? Ford in 1976? Nixon in 1968?
Also, many liberals in 2000 did NOT vote for the guy with the "D" next to his name. They voted for Nader. Dumb, dumb move on their part -- but I'm glad they did such a foolish thing.