PHILADELPHIA - President Obama tried to recapture some of the big-stage excitement of his 2008 campaign Sunday, imploring voters not to reward what he called Republican cynicism and incompetence by sitting out the Nov. 2 elections.

The president delivered one of his most stinging indictments yet of the GOP record to thousands of people in Philadelphia's Germantown neighborhood.

"It's going to take us a while to get out of this hole," he said of the economic malaise that he blamed on GOP policies in place before he took office.

And, saying that "the American people deserve to know who is trying to sway their elections," he suggested that funding for Republican campaigns may be coming from overseas.

"You don't know," Obama said. "It could be the oil industry. It could even be foreign-owned corporations. You don't know because they don't have to disclose."

Similarly, Obama adviser David Axelrod said Sunday on CBS's "Face the Nation" that secret political donations to the U.S. Chamber of Commerce and other groups pose "a threat to our democracy."

The chamber has vehemently denied the allegations, characterizing them as part of a desperate strategy to stave off a GOP takeover of Congress. The business lobby has vowed to spend up to $75 million on the midterm elections, primarily in favor of Republicans.

Chamber Senior Vice President Tom Collamore called the Democratic attacks "a blatant attempt to avoid a serious discussion of Americans' top priority -- creating jobs and growing the economy."

In his speech, Obama acknowledged that many Americans are frustrated, especially about the economy. But he said Republicans have decided "to ride that frustration and anger" without providing solutions. And he said congressional Republicans deliberately opposed his administration on almost every issue.

"If I said there were fish in the sea, they said 'no,' " Obama said. "They figured 'if Obama fails, then we win.'"

His speech was delivered at the second of four big rallies designed to spur Democratic turnout. The first was on Sept. 28 at the University of Wisconsin campus in Madison.

Obama was introduced Sunday by Vice President Joe Biden, who was born in Scranton, Pa.

The Associated Press and Washington Post contributed to this report.