Brown, who'd been targeted by reform groups to support the Disclose Act, a Democratic-crafted bill in response to a Supreme Court case freeing up corporate and labor spending in elections, said the bill does not do enough to ensure fair elections.

"As stated in your letter, my election to the U.S. Senate sent a message that the American people are tired of the politics-as-usual mentality, and want to restore real checks and balances in Washington," Brown wrote in response to the groups. "Unfortunately, the Disclose Act does not do enough to require transparency, accountability and fair play. Therefore, I cannot support the Disclose Act."

Democrats had hoped that Brown would provide key support to move forward with the legislation, which has been criticized by many Republicans as an infringement upon free speech. With Democrats in control of only 59 Senate seats, they need at least one Republican to reach the 60 votes necessary to overcome a filibuster and move forward with the bill.

Brown was targeted for the centrist record he's maintained since joining the Senate in January. While he's stuck with Republicans on most issues, he's supported some extensions of unemployment insurance and will support Wall Street reform legislation on final passage.

But the Massachusetts Republican condemned the bill as advancing "the political agenda of the majority party and special interests in an effort to gain a tactical and political advantage little more than 100 days before an election."

Brown signaled he'd favor a more comprehensive campaign finance reform effort in the vein of McCain-Feingold instead.

Democrats have fallen behind schedule with their push to overhaul camapaign finance rules. Sen. Charles SchumerChuck SchumerRomney: I never got a call from White House to discuss voting rights Kyrsten Sinema's courage, Washington hypocrisy and the politics of rage Joe Biden's disastrous 48 hours MORE (D-N.Y.), the key sponsor of the legislation in the Senate, had hoped to bring it to the Senate floor before the Fourth of July recess.