Okay, fair enough. Romney is saying he's a Republican, but given the chance to vote against Bill Clinton in 1992, he decided to meddle in the Democratic primary. If he'd had the opportunity to vote for a Republican in the Republican primary, he'd have jumped at the chance, but lacking that, he voted Democratic.
Seems reasonable, except there was a Republican primary. But it's not just the facts that get in the way of Mitt Romney's explanation. It's Mitt Romney himself, because just about each time he's explained the vote, he's explained it differently.
On December 15, 1993, The Boston Globe reported that Romney said "he couldn't recall" for whom he had voted. Six weeks later, the Globe reported that Romney "confirmed he voted for former US Sen. Paul Tsongas." Why? Because "favored his ideas over those of Bill Clinton." Then in October of 1994, The Washington Post reported that Romney "publicizes his brief stint as a Democrat to support ex-senator Paul Tsongas in the 1992 presidential primary."
Flash forward 13 years to February, 2007 and Romney offered a completely different rationale in an interview with ABC's George Stephanopoulos: "When there was no real contest in the Republican primary, I’d vote in the Democrat primary, vote for the person who I thought would be the weakest opponent for the Republican."
So Romney has gone from not being able to recall who he voted for, to saying he voted for Tsongas because he liked Tsongas, to saying his vote for Tsongas shows he is bipartisan, to saying he voted for Tsongas to oppose Bill Clinton, to saying he voted for Tsongas to weaken the Democrats, to saying he only voted for Tsongas because he didn't have a chance to vote for a Republican. Except, of course, he did.
So what does Mitt Romney really believe? Nobody really knows. And that probably includes Mitt Romney himself.