Whatever the reason for the shifts in polls, they have rocked the Romney campaign in states such as Virginia, which Romney badly needs to return to the Republican column. Until Obama's win in 2008, Virginians had not rejected a GOP presidential nominee since 1964.
A Washington Post poll of likely Virginia voters showed Obama leading by 8 percentage points, while polls by Fox News and Quinnipiac/CBS/New York Times each showed Obama with a 7 percentage point lead.
Republican campaign strategist Chris LaCivita of Virginia said the polls seem to be projecting a larger Democratic turnout than will materialize. While early voting will have begun in 30 states by the end of this week, Republicans in Virginia and other states promote their turnout machines as the keys to close states.
"Everything I see continues to show an extremely competitive race ... won or lost in the last 72 hours," LaCivita said.
But Steve Jarding, a veteran Democratic strategist in Virginia, says Romney hurt himself with talk of steep cuts in government programs, a threat to the many thousands of federal workers in the Washington suburbs. "They take pride in what they do," Jarding said.
The Romney campaign still has time, through what it calls a sophisticated system of targeting, to identify swing voters, especially after the debates. His advisers hope the three debates will let them reset the campaign after what they acknowledge has been a difficult stretch.
"Forty-some days, that's a lifetime," said Rich Beeson, Romney's political director.
Obama, Romney and groups that support them have poured millions more dollars into television advertising in Florida, Ohio and Virginia, doubling their total spending since early September to nearly $10 million last week in each of the three, according to reports of ad spending provided to The Associated Press.
Obama's spending has flattened in North Carolina, where some Democrats agree that Romney has a slight edge. Romney and Republican groups were outspending Obama there last week by $2 million to about $680,000.
Both campaigns have poured advertising money into Wisconsin in the past two weeks. Obama made his first buys there last week and has spent more than $2 million since. Romney, who had hoped to put running mate Paul Ryan's home state into play, has contributed heavily to almost $5 million in GOP spending there since early September.
An NBC poll showed Obama leading Romney by 5 percentage points in Wisconsin, and by 8 percentage points in Iowa.
"All has been slipping," said Iowa Republican Doug Gross, Romney's 2008 campaign Iowa co-chairman. "We are no exception."
In the race for the 270 electoral votes need to win, Florida (29 votes) is always the biggest up-for-grabs state, and this year it seems to hold special promise for Romney.
Unemployment there still exceeds the national average, helping his indictment of Obama's economic performance. The housing collapse has left vast numbers of homeowners in default.
Yet two polls of likely Florida voters, one by Fox News and one by NBC, showed Obama leading 49 percent to 44 percent.
The storm-delayed GOP convention in Tampa didn't rally Florida Republicans as they hoped, said Florida Democratic Party Chairman Rod Smith. Also, voters are starting to see glimmers of economic hope, he added. "I'm not saying the sun is up, but people can see that it's coming," he said.
Smith noted that Florida's new voter ID law, pushed by GOP lawmakers, may suppress the vote among some Democratic-leaning constituencies.
Palm Beach County Republican Chairman Sid Dinerstein expressed confidence. "We're very happy, very optimistic," he said.
But there is growing concern for Romney in Ohio, where no Republican has lost and been elected president, and where a Fox News poll showed Obama with a 7 percentage-point lead.
"I'd be worried if the election were held today," said Ohio Republican Chairman Rob Bennett.
Babington reported from Washington. Associated Press writer Julie Pace in Washington contributed to this report.