Maryland Democratic Gov. Martin O'Malley and Virginia Republican Gov. Bob McDonnell clashed on national television Sunday, each battering the other on tax returns, Medicare and presidential campaign rhetoric in a reflection of the increasingly bitter and nasty race for the White House.

O'Malley and McDonnell, leaders of their respective governors associations and top surrogates for President Obama and Republican challenger Mitt Romney, echoed the attacks of the presidential contenders, focusing entirely on national rather than local politics on NBC's "Meet the Press."

Among Obama's most vocal supporters, O'Malley homed in on Romney's refusal to release more than two years' worth of tax returns, saying the former Massachusetts governor's assurances that he paid at least a 13 percent tax rate each of the last 10 years was not enough to satisfy voters.

"He has been engaged in tax-avoidance schemes with offshore accounts in the Cayman Islands and the Bahamas," O'Malley said, accusing Romney of "hiding his money in offshore accounts and betting against the future of the United States."

McDonnell countered that the Democratic focus on tax returns was part of a broader attempt by the Obama campaign to distract the public from the president's inept handling of the economy. He lambasted recent remarks from Vice President Joe Biden in which the former Delaware senator told a largely black audience that Romney and Republicans wanted to "put y'all back in chains."

"This is way over the top," McDonnell said. "These character attacks by the other side are just horrific. It's way beneath the dignity of the American people."

The Virginia governor also defended the Republican position on Medicare, which has received greater scrutiny in the wake of Romney's selection of Wisconsin Republican Rep. Paul Ryan -- and not the Virginia governor -- as his running mate.

"We're broke, and they're being honest about the fact that we're broke," McDonnell said, adding that the entitlement program for seniors would become insolvent without reforms.

In contrast, O'Malley said the Ryan proposals embraced by Romney would force seniors to pay more money into a voucher-style system that would reduce coverage for aging Americans during their golden years.

And O'Malley, framing the election as a clear choice between two polar-opposite visions, called Romney hypocritical for bemoaning the harsh tone on the campaign trail.

"Gov. Romney's the sort of guy that you would never want to play pickup basketball with," O'Malley said. "He's always fouling and he's always crying foul."