Top surrogates for President Barack Obama and Mitt Romney traded blame on Sunday for the increasingly negative bent of the race for the White House.

Virginia Gov. Bob McDonnell (R) decried "horrific" character attacks on Romney, the presumptive Republican presidential nominee, while Maryland Gov. Martin O'Malley (D) argued a "very clear contrast" between Obama and Romney has emerged since Wisconsin Rep. Paul Ryan was named to the GOP ticket last week.

Govs. Martin O'Malley and Bob McDonnell visit Meet the Press to discuss the latest developments in the presidential race, including the impact Paul Ryan has had on the debate.

"This is a serious election, and it calls for serious candidates that have real solutions," McDonnell told moderator David Gregory on "Meet the Press." "The time for rhetoric is over."

Romney selected Ryan as his running mate last Saturday, passing over McDonnell, among a short list of other Republican vice presidential hopefuls.

But while that selection was heralded just a week ago as one directed toward injecting the 2012 campaign with substance, and serious debate about big issues, the week since then has seen some of the most negative rhetoric of this election.

The Mitt Romney campaign is accusing Vice President Joe Biden of using "fear and smear" tactics. NBC's Kristen Welker reports.

Just this past week, Obama joked about Romney's having put his family dog on the roof of the car during a long road trip, while Vice President Joe Biden told a mixed race audience that, by removing regulations, Republicans and Romney want to "put y'all back in chains." That language prompted Romney to accuse the president of running a "campaign of division and anger and hate."

"It certainly was an indelicate choice of words," O'Malley said of Biden's "chains" comment.

But the Maryland governor — who, like McDonnell, is considered a very early and preliminary potential candidate for president in 2016 — argued that it's especially ironic for Romney to bemoan a negative tone in the campaign.

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

This week's sniping extended to the issue of taxes, too, after Romney inadvertently revived scrutiny of his personal taxes by saying he paid "at least" 13 percent in federal taxes over the past decade. That admission prompted new demands that Romney release additional tax returns than what he has released and pledged to release.

"This is what we know. We know that he has engaged in tax avoidance schemes, with offshore account in the Caymans and the Bahamas," O'Malley said.

Romney and Obama may have had quiet Sundays, but their surrogates hit the morning talk shows, debating Medicare and health care reform. NBC's Kristen Welker reports.

McDonnell called the focus on taxes "diversionary" in defending Romney's decision to release only his 2010 tax returns and his forthcoming 2011 returns. Voters, he said, would rather focus on the economy, jobs, or Medicare.

"These are the substantive issues Americans care about; not tax returns," the Virginia governor said.

The way both surrogates traded blame was emblematic of the 2012 presidential campaign, in which both the Romney and Obama campaigns have fought daily battles from the trenches, squabbling publicly over an issue set that can seem trivial sometimes. The Obama campaign in particular has sought to turn the election into a "choice" between the president and Romney, versus a simple referendum on Obama's own policies.

To that end, Texas Republican Senate candidate Ted Cruz, a favorite candidate of the Tea Party movement, argued that if the campaign is about issues, the GOP would win in November.

But, Cruz cautioned: "If it's a battle of personalities, Republicans will lose."