The Romney campaign circulated material during the day that quoted Obama, then a state senator in Illinois, as saying he "probably would have voted against it" if he had been in Congress.
The Obama administration recently announced plans to issue waivers to states that wanted "to test alternative and innovative strategies, policies and procedures" to improve employment among needy families. It said it was acting after receiving requests from some of the nation's governors, including Republicans in Utah and Nevada. But senior GOP lawmakers attacked the move as an attempt to undermine the welfare-to-work requirements in effect for more than a decade.
In his statement, Clinton said there would be no waiver of time limits, which he called an important feature of the 1996 law, under Obama's plan. "The Romney ad is especially disappointing because, as governor of Massachusetts, he requested changes in the welfare reform laws that could have eliminated time limits altogether," Clinton said. "We need a bipartisan consensus to continue to help people move from welfare to work even during these hard times, not more misleading campaign ads."
Officials with access to detailed advertising information said it appeared the commercial was airing at heavy levels in Colorado, Florida, Iowa, North Carolina, New Hampshire, Nevada, Ohio and Virginia — states where the race is closest.
Romney was himself targeted with a new ad during the day, this one launched by Priorities USA Action, a super PAC that supports Obama.
It features a former Kansas City steelworker who says his company was taken over in 1993 by a group that included Bain Capital, the private equity firm co-founded by Romney.
"When Mitt Romney and Bain closed the plant, I lost my health care, and my family lost their health care," he said. His wife became ill, but "I think maybe she didn't say anything because she knew we couldn't afford the insurance," he says. By the time she went to the hospital, she was diagnosed with cancer and died quickly, he said.
"I do not think Mitt Romney realizes what he's done to anyone, and furthermore I do not think Mitt Romney is concerned."
In response, Ryan Williams, a spokesman for Romney, said Obama's allies "continue to use discredited and dishonest attacks in a contemptible effort to conceal the administration's deplorable economic record."
The timeline for Romney's role at Bain, the plant's closing and the woman's death raises questions about the legitimacy of linking the events.
Romney left Bain Capital in 1999 to lead preparations for the 2002 Winter Olympics. Although he retained his official standing as Bain's founder and chief executive until 2002, his campaign says he didn't have a managerial role. The plant closed in 2001 and the steelworker's wife died in 2006.
As the party conventions neared, Republicans and Democrats were fleshing out the speaking schedules for their gatherings.
Republicans said Romney's most persistent primary rival, former Pennsylvania Sen. Rick Santorum, would have a turn at the speaker's podium. So, too, former Florida Gov. Jeb Bush and Kentucky Sen. Rand Paul, a tea party favorite.
Former President Jimmy Carter will tape a video message to be aired in prime time at the Democratic convention.
Thomas reported from Washington. Associated Press writers Julie Pace, Ricardo Alonso-Zaldivar, Stephen Braun and Kasie Hunt in Washington contributed to this report.