WASHINGTON — Maine’s two senators joined their Republican colleagues in blocking action on a bill that would require outside groups spending hundreds of millions on campaign ads to reveal how much they spend and who their big donors are.

With Republicans united in opposition, Democrats had no chance of getting the 60 votes needed to advance the Disclose Act to the full Senate.

The bill would have required groups spending more than $10,000 during an election cycle to file a report within 24 hours and identify those donating more than $10,000. Current election law requires super political action committees, or super PACS, to make periodic reports to the Federal Election Commission, but nonprofits, including social welfare organizations, labor unions and trade groups, generally do not have to reveal the sources of election-related spending.

Democrats accuse the GOP of attempting to shield big-money donors from disclosure requirements.

Maine Sens. Susan Collins and Olympia Snowe said they support full financial disclosure but found fault with a bill that they said favored unions and was being rushed through Congress without hearings or opportunity for amendment. Both Snowe and Collins supported the McCain-Feingold bill, the bipartisan campaign finance law that was changed by the Citizens United decision.

Collins said Citizens United created serious problems but that the Democrats’ bill “preserves certain advantages for labor unions.”

“What we need is legislation that is truly equitable, that treats corporate and labor interests in the same way and that ensures there is disclosure of major donors,” Collins said in an interview prior to the Senate vote.

Snowe offered similar reasons for her votes against the bill.

“I am concerned about the legislation’s clearly unequal treatment of unions and corporations, as it imposes a new donor disclosure regime on certain entities, while not on others,” Snowe said in a statement.

The Sunlight Foundation, a nonprofit and nonpartisan group that works for greater transparency in government, disputed that the bill favored unions.

– The Associated Press contributed to this report.

Washington Bureau Chief Kevin Miller can be contacted at (207) 317-6256 or at: [email protected]

Twitter: @KevinMillerDC