Senate campaigns also have been affected, notably in Ohio, where Democratic Sen. Sherrod Brown's commanding lead in the polls began to erode after the U.S. Chamber of Commerce and others started a televised attack in the spring. Overall, GOP-aligned organizations have spent roughly $30 million on ads in key races, compared with about $11 million for groups supporting Democrats.
Underscoring the concern, Rep. Steve Israel of New York, who heads Democrats' efforts to regain House control, issued a thinly veiled call for his party's donors to step up. The recent recall election in Wisconsin "should serve as a wake-up call," he wrote, referring to the lopsided advantage in spending by outside groups that helped GOP Gov. Scott Walker overcome a union-backed bid to oust him.
Other Democratic efforts to catch up are less publicized, particularly when it comes to Priorities USA Action, the group formed to boost Obama's reelection.
David Axelrod, a top strategist for the president, is expected to meet with potential donors to the group in New York on Monday, according to officials familiar with his plans. Separately, former President Bill Clinton has agreed to help, although it isn't clear whether he will appear at a formal fund-raising event.
Outside groups have allowed Romney to remain competitive in the television ads wars while restocking a treasury that was depleted during the battle for the Republican nomination. It also raises the possibility that Obama, the Democratic Party and allied groups will be outspent by a combination of Romney, the GOP and allied organizations, erasing an advantage the president had in 2008.
Earlier this year, Obama's campaign decided to dip into its own treasury to respond to commercials from the American Energy Alliance, which had spent more than $3 million attacking the president.