That may be true. But you're talking about a general election between the Republican and Barack Obama. Santorum's endorsement of Mitt Romney came during the primary. If he did not think that Romney was a "conservative" by his own standards there was no need to endorse him or McCain when he did.
Nobody in their right mind actually believes Santorum's sudden acknowledgment that Mitt Romney is a moderate Republican after he once claimed Romney was a "real conservative". It just looks fake when he talks about Romney like that now. It's true Romney is a liberal republican. It's not true that Santorum really believes it. Santorum wants to be President.
You need to put things in context.
In a race between Mitt Romney and John McCain for the 2008 GOP Nomination, who would you call the “true conservative” of that race after it became clear it was a two man race?