Pelosi has flatly denied the incident ever occurred.
"Clearly, this ad is an act of desperation," she said in a statement released Sunday.
While national polls remain tight, polls in several of the most closely watched states, including Colorado, suggest that Obama has opened narrow leads. Obama won Colorado by 9 points four years ago, but the state went to a Republican in the previous three presidential elections.
Amid mounting pressure to spend less time raising money and more time explaining his plans to voters, Romney was refocusing his schedule visit more frequently the most competitive states. Romney adviser Kevin Madden defended the fundraising focus as a necessity, but said that intensity would be matched by an aggressive schedule of public events starting Sunday.
From Denver, Romney was to begin a three-day bus tour in Ohio on Monday followed by a stop in Virginia — states that Obama won in 2008 but that Republicans claimed four years earlier. Obama, meanwhile, was set to be in New York on Monday and Tuesday for a speech before the United Nations General Assembly, plus an appearance on ABC's "The View."
Lederman reported from Washington. Associated Press writer Jim Kuhnhenn in Milwaukee contributed to this report.
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