Will Romney move to the middle? Can he?

The playbook for Republican presidential contenders goes at least as far back as Richard Nixon: Run hard to the right in the primaries; steer back to the center for the general election.

Former Massachusetts governor Mitt Romney has sold himself all along as the most electable Republican in the field. But as he moves closer to becoming the GOP standard-bearer, the candidate whose authenticity has been questioned from all sides faces a tricky challenge making a pivot for the next phase of the race.