Senate Republicans on Thursday successfully blocked a key vote on repealing “Don't Ask, Don't Tell.”

The Senate's GOP caucus, with the exception of Maine Senator Susan Collins, united to filibuster the National Defense Authorization Act, which includes an amendment to end the law that bans gay and bisexual troops from serving openly.

Moderate Republicans who had signaled they might cross the aisle – including Lisa Murkowski of Alaska, Scott Brown of Massachusetts and Olympia Snowe of Maine – did not follow Collins' lead.

Aubrey Sarvis, executive director of Servicemembers Legal Defense Network (SLDN), the largest group lobbying for repeal, called the 57-40 vote an “outrage.”

“Today a band of Senators voted to continue the discrimination against gay and lesbian service members who are fighting and dying for our country,” Aubrey Sarvis said in a statement. “This continued delay is an outrage against these service members and the more than 14,000 who have already lost their jobs under this discriminatory law.”

“History will hold these senators accountable and so will many of their constituents,” he added. “There will be no place for Senators to hide.”

Senator Blanche Lincoln, a Democrat from Arkansas who voted against repeal in September, arrived late for the vote, but told reporters, “Had I been here I would have voted yes.”

Republicans on Wednesday threatened to filibuster the measure unless Reid allowed for greater freedom to choose amendments and up to 34 hours of debate on the bill.

“It's clear that Republicans don't want to hold a vote on 'Don't Ask, Don't Tell,” Reid said. “They want to block a bill on this at all costs, even if it means not passing the defense authorization bill for the first time in 48 years.”

After the vote, Alex Nicholson, executive director of Servicemembers United, called on Congress to approve repeal of the law as a stand-alone bill.

“Since the votes are there in isolation, the Senate should still consider a stand-alone bill to repeal the 'Don't Ask, Don't Tell' law before adjourning for the winter holidays,” he said in a statement.