With a key vote on whether to end debate on an annual defense policy bill that includes repeal of “Don't Ask, Don't Tell” scheduled for Tuesday, repeal advocates are insistent the measure must pass before the midterm elections.

Republicans, led by Arizona Senator John McCain, are expected to filibuster the bill when it comes to the Senate floor.

Advocates for repeal have been lobbying moderate GOP members, including Senators Susan Collins and Olympia Snowe of Maine, George V. Voinovich of Ohio and James Webb of Virginia, to oppose the filibuster.

Lady Gaga headlined a rally in Portland on Monday to call on Maine senators to support the measure.

“If you are not honorable enough to fight without prejudice, go home,” she told a crowd at the #4THE14K rally held near the University of Southern Maine.

Collins, however, rejected the plea, signaling on Monday that she's likely to support the filibuster. The 57-year-old politician said she objected to Senate Majority Leader Harry Reid's plan to limit GOP amendments, saying the Democratic leadership of the Senate “intends to shut Republicans out of the debate.” Similarly, Snowe, a former first lady of Maine, appeared on track to side with her GOP colleagues.

Other Republicans have also seized on the idea, saying Reid, a Democrat from Nevada, is playing politics with the bill in a desperate effort to shore up votes as he faces a tough reelection fight against tea party favorite Sharron Angle, who is vehemently opposed to gay rights.

“I understand that it's not that far between now and elections, but to use a bill that has to do with defending our national security interest when we're in two wars to pursue a social agenda and a legislative agenda to galvanize voting blocs I think is reprehensible,” McCain said.

Repeal advocates concerned that a GOP takeover of Congress would kill the measure, insist repeal of “Don't Ask” must be approved before the elections.

“The simple fact is, after the election there would be reluctance to take on this issue,” Christopher Neff, deputy executive director of the Palm Center, a think tank at the University of California at Santa Barbara that lobbies for repeal, told the Washington Post. “”We've lined up the stars. This needs to happen now.”