The battle over Mitt Romney's tax returns has returned to the campaign fray with new information from Mitt and a new challenge from the Obama camp.

On Thursday, Romney said he paid at least 13 percent of his income in taxes in each of last 10 years.

The Associated Press, citing the nonpartisan Joint Committee on Taxation, reports middle-income families making $50,000 to $75,000 pay an average of 12.8 percent.

Romney's new claim is no doubt aimed at blunting the claim (offered without a shred of evidence) by Democratic Senate Leader Harry Reid that Romney paid no income taxes for 10 years.

So at first blush, the new Romney tax info would seem to help Mitt. But by reprising the issue Romney handed Obama's camp a new opening, which it quickly took.

On Friday, Obama campaign manager Jim Messina released a letter to the Romney campaign saying it promises to drop the issue if Romney releases five years worth of returns.

"Gov. Romney apparently fears that the more he offers, the more our campaign will demand...I am prepared to provide assurances on just that point: if the Governor will release five years of returns, I commit in turn that we will not criticize him for not releasing more -- neither in ads nor in other public communications or commentary for the rest of the campaign," the letter says.

The five years includes 2010, which Romney has released, and 2011, which he has pledged to release.

The Obama tact is politically smart because it keeps the issue alive; but it's also disingenuous since Romney and his wife repeatedly and firmly say they plan no further tax releases, and to change that position now would add weight to Mitt's flip-flop image.

The new proposal and its all-but-certain rejection also could fuel further speculation about why Romney is withholding the returns. This includes possibly revisting a report last month in Mother Jones magazine that Romney's company, Bain Capital, invested in a medical-waste disposal company, Stericycle, that, among other things, disposes of aborted fetuses collected from family planning centers.

Whether Romney's income or management of Bain can be directly linked to Stericycle or not, just raising the issue of a possible connection could cause problems for the GOP candidate.

How far any of this goes is anyone's guess. But I'm guessing Obama will press the tax button from here to November 6.