I assume this new ad from the Romney campaign...

...has something to do with poll numbers like this, from Rasmussen:

The race for the Republican presidential nomination is now nearly even with Mitt Romney still on top but Newt Gingrich just three points apart.

Romney's nomination remains all but inevitable. Nate Silver says Romney's odds of capturing the nomination are at least 90 percent, maybe better.

But perhaps Gingrich drags it out a while longer?