Washington (CNN) -- Pop star Lady Gaga will headline a rally in Portland, Maine, on Monday calling on the state's two Republican senators to vote yes on taking up a defense bill that includes authorization to repeal the military's "don't ask, don't tell" policy toward gay and lesbian servicemembers.
A crucial Senate vote is scheduled for Tuesday afternoon, and supporters of the repeal say that as of now, they do not have the 60 votes needed to overcome a GOP filibuster blocking the bill.
Maine Republicans Susan Collins and Olympia Snowe are publicly undecided, and could each potentially provide Democrats the votes they need to bring the issue before the Senate.
Lady Gaga's rally is being organized by Servicemembers Legal Defense Network (SLDN), a gay rights group mobilizing grassroots support to repeal the controversial policy. The organization also helped the singer set up a lengthy YouTube video posted Friday urging her fans to call their senators and ask them to support Tuesday's procedural vote.
The singer sent word of the planned rally via twitter Sunday night.
A spokesman for SLDN says Gaga will be accompanied at Monday's rally by both gay and straight military veterans. Both of Maine's GOP senators have been generally supportive of the gay rights community, and this issue in particular.
Collins voted to authorize the Pentagon to repeal the don't-ask-don't-tell policy when the issue was before the Senate Armed Services Committee this summer, and SLDN executive director Aubrey Sarvis calls both Collins and Snowe "key to breaking the filibuster" on Tuesday.
Republican leaders are trying to keep all 41 GOP senators on board to block the bill, arguing that Senate Majority leader Harry Reid, D-Nevada, is unfairly limiting the amendments allowed to try to change the bill once it is on the Senate floor.
GOP leaders are also ridiculing Reid for trying to bring what they call a purely political vote to the Senate floor right before the election, to appease the gay community, which has been frustrated with Democrats for inaction on this issue.
On Friday, aides to both Collins and Snowe told CNN they were still undecided about how they would vote. GOP leadership sources told CNN they feel confident they can keep all their members in line and prevent the defense bill from coming to the Senate floor.
Lady Gaga's rally will take place at 4 p.m. ET at Deering Oaks Park in Portland, Maine, near the University of Southern Maine campus.