There's little doubt Romney can repair some of this damage. A small but significant amount is likely driven by intra-Republican opposition from the primary race itself. Santorum's and Gingrich's supporters aren't crazy about Romney at the moment. And once Romney is the nominee a good bit of that animosity will soften.
But fundamentally Mitt Romney's reputation has taken a terrible beating across the spectrum, especially with independent and loosely affiliated voters. (He's actually hitting almost Gingrich levels of unpopularity. He's at -24 and Newt's at -36.9.)
So when does he start to repair the damage? Ideally, right now. But that's the problem. Not only is the race not sewn up; the stitches seem to get looser by the day. Romney is now running behind Santorum nationwide and in the Mitt-critical state of Michigan.
Yesterday or today I heard someone on TV say that Super Tuesday is Romney's last chance to take the nomination as the consensus candidate. Here 'consensus candidate' would mean that the party decides to rally around a 'winner' well before that person is anywhere near to mathematically clinching the nomination. The other alternative is to slug it out for months and take it by (electoral) brute force.
And this is where we come back to favorability ratings. I doubt there's any way Romney really repairs the damage to his reputation and favorability before he gets the nomination fight behind him. There are two big factors working against him.
First, the intensity of the primary fight is forcing Romney to take increasingly hard right positions that are alienating independent voters. As much as it's a cliche, he can't effectively tack to the center until he's got the primaries behind him.
Second, and a bit more intangibly, running around the country in a long twilight struggle with Rick Santorum is just ... how to put it? inherently demeaning and diminishing. It's like struggling to land a one pound fish or searching for the way out of a paper bag. People see you doing that and you just look weak and feckless, even pitiful.
So how long does this play out? If Romney can get Michigan in hand, win convincingly and then do the same on Super Tuesday it's probably back to being over. He'll still have his work cut out for him but by mid-March he can get back into general election mode.
But what if he doesn't? The polls say it will be a challenge. And the deeper this gets into the Spring the closer you get to the point on the calendar where Romney simply won't have enough time on the clock to undo the damage.