Could "liar, liar" backfire?

The Obama campaign's feverish bid to cast Mitt Romney as "dishonest" on his tax-plan claims is running into headwinds ahead of the vital second presidential debate.

Fact-checkers have been heavily scrutinizing the Obama campaign's logic. One economist chided the team for using his research to claim Romney's pushing a middle-class tax hike. And Republicans claim the "liar, liar" strategy is a sign of desperation after a tough debate, with just four weeks left on the clock until Election Day.

"This business of just going out and calling a candidate for the president of the United States a liar, just saying liar, liar -- that in my view ... this is a new low," Sen. John McCain, the 2008 Republican nominee, told Fox News on Tuesday. "It means that they're out of ideas."

Ann Romney, in an interview with Fox News, also questioned the Obama campaign claims.

"This is what (Romney) believes. These are the policies. These are his statements. I mean, it's sort of like someone in the sandbox who lost the game and kicks sand in someone's face and says 'you liar'," she said. "To me it's poor sportsmanship."

The campaign is not quite calling Romney a liar, though it's come close.

Obama advisers and President Obama himself insist that Romney last week was peddling a platform that diverges from what he's been selling on the stump these past many months. Central to that argument is their claim he wants a $5 trillion tax cut, which Romney denied last week.

The Obama campaign argues that while Romney says his plan is deficit-neutral, there's no way he'd be able to find enough loopholes and other benefits to sacrifice in order to make up the difference. Therefore, Obama says, the plan would end up either raising taxes on the middle class or adding to the deficit.

The Obama campaign launched an aggressive effort to cast Romney as untruthful in the aftermath of the debate.

Adviser Robert Gibbs told "Meet the Press" on Sunday that Romney will "say anything to get elected."

A new Obama TV ad declared Romney's tax claims were "dishonest."

But fact checkers returned fire. The AP said Obama's claim "doesn't add up." Politifact deemed the president's claim "half-true," calling it "accurate but misleading." Could Romney's tax cuts by themselves add up to $5 trillion? Maybe. But he also pledges to close loopholes and deductions to make up the difference, though he hasn't offered many specifics.

Obama acknowledges this, but still calls it a $5 trillion tax cut. "The $5 trillion figure accounts for only half of Romney's plan," Politifact notes.

The campaign on Sunday released a breakdown of how it got to the $5 trillion number. It counted $2.5 trillion from Romney's plan to lower rates by 20 percent across the board. It included $1.1 trillion from a corporate tax cut, and $150 billion from a repeal of the estate tax, among other things.

"Even the studies that Romney has cited to claim his plan adds up still show he would need to raise middle class taxes," the campaign said in a statement -- claiming Princeton economist Harvey Rosen found "paying for Romney's tax cuts would require large tax increases on families making between $100,000 and $200,000."

Rosen, though, told The Weekly Standard that he "can't tell exactly how the Obama campaign reached that characterization of my work."

His study took into account "additional income" that could be generated by economic growth under the plan. Rosen wrote in the study that "under plausible assumptions," a Romney-style proposal "can both be revenue neutral and keep the net tax burden on high-income individuals about the same. That is, an increase in the tax burden on lower and middle income individuals is not required in order to make the overall plan revenue neutral."

The Obama campaign's math also includes corporate tax cuts -- though Obama, too, has called for a corporate tax cut. The major difference between them is just that Obama wants to bring the top rate from 35 to 28 percent, while Romney wants to bring it to 25 percent.