The Obama campaign and Democrats are continuing an effort to smear Mitt Romney's credibility with two new ads that suggest the presumptive GOP nominee has used "every trick in the book" to avoid paying U.S. taxes.

"Makes You Wonder," a 30-second spot that begins airing today across Pennsylvania where Romney is holding a rally, seizes on Romney's offshore finances and limited release of his tax returns: "Makes you wonder if some years he paid any taxes at all," the narrator says. (The only year for which he has released a return, 2010, Romney paid roughly $3 million in taxes, a rate of 14 percent.)

A Democratic National Committee web ad highlights a growing list of Republican voices that have joined the calls on Romney to release more of his returns, surmising that the reason he refuses is because he has something to hide.

"What did the one Republican who had 23 years of Mitt Romney's tax returns do in 2008?" the narrator says. "He chose Sarah Palin. What does John McCain know that the American people don't?"

Romney has thus far released fewer tax returns than any presidential nominee from both parties in the last 30 years. Still, his campaign insists that two years of tax returns are sufficient, claiming the frenzy over Romney's finances is a fabricated and insatiable assault. Aides also believe the negative attacks are not resonating with voters.

"The Obama campaign appears to be stuck in their own 'groundhog day,' repeating the same, debunked charges they've waged for weeks in an effort to distract voters from this administration's failure to fix the economy and create jobs," said Romney campaign spokeswoman Amanda Henneberg. "Americans deserve a president they can trust, and not someone willing to say and do anything to win an election."

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Four years ago at the Democratic National Convention, Obama protested the demonizing negative attacks perpetrated in the campaign, explaining "if you don't have a record to run on then you paint your opponent as someone people should run from."

Now he appears to be engaging in the very same practice he once decried, and offering no apologies.

"I think the president is laying out the choice. Elections are about choices. And we have two very different directions that we can go in this country and that's what the president is communicating," deputy Obama campaign manager Stephanie Cutter told NBC's "Today" when asked about the negative tone of their campaign.