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In the Senate race in Ohio, ads financed by the Chamber of Commerce, American Crossroads, Sixty Plus and other groups coincided with a drop in Mr. Brown’s poll ratings, creating at least the appearance of a competitive race against Republican Josh Mandel where little or none had existed. The most recent attack, airing at a cost of $1 million statewide, says the incumbent voted for “every bailout proposed by (George W.) Bush and Obama,” a rare use of the former president’s image or name in the current campaign.

Mr. Brown gained modest support on television from Democratic-aligned groups after he came under attack, and he swiftly began using the disparity to appeal for campaign donations.

Mr. Brown and many other Democratic incumbents hold advantages over their rivals in campaign cash on hand. But a multimillion-dollar disparity in television advertising by outside groups eventually could stretch the Democratic Senatorial Campaign Committee thin at a time it is struggling to defend a narrow Senate majority.

“It’s a source of great disappointment that people who say they’re on our side remain on the sidelines,” Sen. Richard J. Durbin of Illinois, a member of the Democratic leadership, said recently. Mr. Durbin, Majority Leader Harry Reid of Nevada and Sen. Charles E, Schumer of New York have all attended fundraisers for the Majority PAC, an organization established to aid Democratic Senate candidates that reported taking in $6.1 million through May 23.

“There’s some movement in our direction, but not enough” since then, Mr. Durbin added, and other officials said fundraising has picked up in recent weeks.

In the general election race for the White House, television ads designed to aid Mr. Obama totaled about $55 million through the early days of June. Of that, the president’s own campaign spent $44.7 million, more than 80 percent of the total, with $9.3 million from Priorities USA Action.

The situation was reversed among Republicans, where outside groups put up about $37 of $44 million spent so far on television ads, or more than 80 percent of the GOP total. Mr. Romney’s campaign has spent about $7.8 million.

The Republican advantage comes at a time the system for financing of campaigns is in flux.

Recent Supreme Court rulings mean that corporations and unions may donate unlimited amounts to political groups. The rules are so complex that some organizations must disclose their donors’ names and the amount of their contributions, while others are not. Some television purchases by independent organizations must be reported to the Federal Election Commission with 48 hours, some within 24 hours, and others not at all.