Would Mitt Romney raise taxes on the middle class? It's become one of the central issues of the campaign. 

Tonight, President Obama is pressing that charge, based on a study released in August by the nonpartisan Tax Policy Center, which concluded that meeting all of Romney's goals for tax reform is "not mathematically possible."

Conservatives have pushed back in recent weeks, charging that the Tax Policy Center concluded that Romney would whack the middle class rather than taking some other, less painful steps, like taxing the interest on state and local bonds or the build-up of value inside life insurance policies.

But some ideas were less helpful to making Romney's case, such as economist Martin Feldstein's suggestion that Romney simply redefine the middle class to include only those making less than $100,000 a year rather than those making less than $250,000. 

Some conservatives complain Romney has gotten himself into this box by failing to release more details about his plan for tax reform. He has offered no new information yet tonight.