Run ads like this:
Video via William Jacobson, who revisits the successful campaigns by Ted Kennedy and John McCain against Mitt:
You can pretty much disregard current polling showing Romney the most competitive with Obama. Romney has not survived these lines of attack in the past, so we should be suspicious that he can do so this time around. If Romney is the nominee, expect these themes to be hammered relentlessly. And expect his negatives to be as high as any other Republican contender.
Michael Walsh explains that, unlike Obama, Mitt lacks a solid base of support:
Head-to-head with Romney next year, Obama will sink him handily. … What can be done at this late date, I have no idea. And neither do the Republicans.
Meanwhile, Politifact has scored Romney's first anti-Obama ad as "pants on fire" mendacious.
The Dish 