Super PACs and other political groups, whose negative ads have become a defining characteristic of the 2012 campaign, merited only a passing mention in Thursday night's vice presidential debate.

The moment came near the end, when moderator Martha Raddatz quoted a complaint from an unnamed soldier: "The ads are so negative and they are all tearing down each other rather than building up the country.”

Radditz asked the candidates if they were ever "embarrassed by the tone" of the contest.

Vice President Biden responded that "there are things that have occurred in this campaign and occur in every campaign that I’m sure both of us regret anyone having said." Then he complained about the ability of super PACs and other outside groups to influence the contest.

"These special new groups that can go out there, raise all the money they want, not have to identify themselves, who say the most scurrilous things about the other candidate, " Biden said. "It’s — it’s an abomination."

Ryan essentially ignored the tone question, pointing to "broken promises" of the Obama administration.

Both Biden and President Obama have regularly complained about the impact of well-funded outside groups on U.S. politics. But their own campaign has been bolstered by a super PAC, Priorities USA Action, that has run millions of dollars worth of attack ads against Republican nominee Mitt Romney in key swing states.

Buoyed by a 2010 Supreme Court ruling allowing unlimited spending on elections by corporations and unions, outside groups have spent well over $130 million on attack ads since spring in the presidential race, primarily in support of Romney. Super PACs must disclose their donors, but many other political nonprofit groups that do not have to reveal their funding have also run issue ads in the contest.