Washington ? Senate Republicans blocked legislation imposing new restrictions on political activity by special interest groups Tuesday, likely dealing a fatal blow to a drive by the White House and congressional Democrats to rewrite campaign rules in the run-up to the midterm elections.

The 57-41 vote was three short of the total needed to advance the measure, which calls for greater disclosure on campaign advertising funded independently by corporations, unions and other organizations, but included an exemption for the National Rifle Association and a small number of other groups.

Less than 100 days before the elections, the debate was highly political — and the outcome widely anticipated.

Anticipating defeat, Democrats swiftly unleashed a coordinated attack employing one of their emerging campaign themes. Republicans, anticipating big gains in the fall, folded the day’s Senate events into their own election-year argument — that Democrats have been unsuccessful in easing double-digit unemployment.