Brown gained modest support on television from Democratic-aligned groups after he came under attack, and swiftly began using the disparity to appeal for campaign donations.
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 could eventually 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," Illinois Sen. Dick Durbin, a member of the Democratic leadership, said recently. Durbin, Majority Leader Harry Reid of Nevada and Sen. Charles 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, 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 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. 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.
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