By law, candidates' campaigns are not allowed to coordinate with independent groups that are aligned with them.
But most political experts agree that the finer points of campaign law are lost on most voters, who tell pollsters they dislike attack ads yet are known to shift their views of candidates because of them.
There is no dispute about their prevalence.
Obama, Romney and super PACs supporting both men have been running such ads for months at a cost of millions of dollars.
Restore Our Future, an independent group that backs Romney, aired ads during the race for the Republican nomination that first crippled Newt Gingrich and then sent Rick Santorum to the sidelines. The group spent more money on ads during the primary season than Romney's own campaign.
The same group as well as American Crossroads and Crossroads GPS have aired ads critical of Obama in more recent weeks, outspending candidate Romney, who can't fully dip into his campaign treasury until after the GOP convention in Tampa, Fla., this month.
Priorities Action USA went after Romney in the ad that sparked this week's controversy.
In such cases, accuracy is not always observed.
This week, Romney accused Obama in a commercial of planning to "gut welfare reform" by dropping work requirements. Gingrich, now a Romney supporter, said there is "no proof today" of the claim.
Last February, Gingrich, then a presidential contender, asked Georgia television stations to remove an attack ad by Restore Our Future. It accused him of once having co-sponsored legislation that sent $60 million a year to U.N. programs supporting China's "brutal one-child policy." PolitiFact, an independent website, rated the charge as false, but the ad was not taken down.
Obama's campaign last month aired an ad saying that Romney "backed a bill that outlaws all abortion, even in cases of rape and incest." FactCheck.org found that Romney's consistent position at least since 2005, is to oppose abortion except in cases of rape, incest or when the life of the mother is in danger.
In this week's controversy, Priorities USA Action says it has no plans to remove the commercial that prompted Romney's comments to Bennett.
Associated Press writers Kasie Hunt in New York and Steve Peoples and Philip Elliott in Washington contributed to this story. Julie Pace reported from Colorado.
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