This is a favorite meme of the left. In fact, apart from Ace's parody of the same, I've never seen a "lifelong Democrat" claim to be, for the first time ever (!) voting Republican in an election. Probably because it's lame and fools no one.
It certainly won't keep them from trying. Over and over and over again.
The latest example is "lifelong Republican," Tony Dean. Courtesy the Buckeye Firearms Association,
A "lifelong Republican" that is "switching parties to head a Sportsmen for Obama group"? This is an interesting claim, given that Dean was mentioned by the Sioux Falls Argus Leader in 2003 as a possible Democratic candidate for Congress who endorsed Democrat Senator Tim Johnson for reelection just a year earlier.