Washington (CNN) - The outcome of a key Senate vote Tuesday on whether to begin debate on legislation that includes a repeal of the military's "don't ask, don't tell" policy remains too close to call.
Senate Majority Leader Harry Reid, D-Nevada, needs 60 votes to bring the bill to the floor, and Republicans are threatening a filibuster that would prevent the 59-member Democratic caucus from doing so.
The Republicans appear united against the measure, including some GOP senators who favor lifting the Pentagon's requirement that gays and lesbians keep their sexuality a secret.
In particular, the Republican opponents are upset that Democratic leaders so far refuse to allow GOP amendments to the broader National Defense Authorization Act that includes the "don't ask, don't tell" provision.
GOP senators also dislike Reid's plan to add an immigration-related provision to the defense bill. Reid wants to tack on a measure that would provide a path to citizenship for students who are children of illegal immigrants.
"I am opposed to illegal immigration, and I am deeply disappointed that Washington politicians are playing politics with military funding in order to extend a form of amnesty to certain illegal immigrants," Republican Sen. Scott Brown of Massachusetts said in a statement issued hours ahead of Tuesday's vote.
Meanwhile, Maine's two moderate Republican senators have indicated they also will join the filibuster, despite pressure from pop star Lady Gaga to support the repeal of the "don't ask, don't tell" policy.
At a rally Monday in Maine, Gaga called on the state's senators - Olympia Snowe and Susan Collins - to join Democrats in overcoming the expected GOP filibuster attempt. To loud cheers from the crowd, Gaga said she was proposing a new law titled, "If you don't like it, go home," which would remove homophobic straight soldiers from the military instead of gay soldiers.
"If you are not honorable enough to fight without prejudice, go home," she shouted.
Without the support of Snowe and Collins, Democrats are unlikely to muster the 60 votes needed to proceed with debate on the defense authorization plan.
Both oppose the "don't ask, don't tell" policy, and Collins was the sole Republican vote in the Senate Armed Services Committee in support of getting rid of it.
But in a statement issued Monday night, Collins said she would side with the rest of the GOP because the Democratic leadership of the Senate "intends to shut Republicans out of the debate."
Collins said she agreed with the chairman of the Joint Chiefs of Staff, Adm. Mike Mullen, that the law is "simply not fair." However, she said Reid should give Republicans and Democrats "an equal opportunity" to offer amendments to the defense bill.
"Now is not the time to play politics, and I again call on the majority leader to work with Republican leaders to negotiate an agreement so that the Senate can debate the defense bill this week," Collins said.
In a separate statement, Snowe also indicated she would support a Republican filibuster, saying the chamber should be allowed a full debate on the measure. Snowe also questioned why the Senate would vote on repeal before the military has completed its review.
"We should all have the opportunity to review that report which is to be completed on December 1, as we reevaluate this policy and the implementation of any new changes," Snowe's statement said.
The defense authorization act, which is a broad defense policy bill, would not rescind "don't ask, don't tell" until after the Pentagon completes a review of the repeal's impact on the military. The review is due in December and would serve as the basis for necessary certification by the president, defense secretary and chairman of the Joint Chiefs of Staff that the military could handle repealing the policy.
Many Republicans complain that Congress should not step in until after that military review is completed.
Sen. John McCain of Arizona, the top Republican on the Armed Services Committee, said approving a repeal provision before finishing the review process would amount to an insult to military personnel.
McCain also is unhappy about Reid's intention to add the immigration provision.
Reid "is turning legislation on our national defense into a political football," McCain said last week. "Politically controversial amendments are crowding out our limited time to debate actual military and defense-related legislation."
Reid denied last week that his scheduling was motivated by politics.
Defense Secretary Robert Gates and Adm. Mike Mullen, the Joint Chiefs chairman, have said publicly they support repealing "don't ask, don't tell."
The military already has working groups looking at how it would implement the change if ordered. The groups are looking at everything from housing to entitlements, and even personal displays of affection.
- CNN's Chris Lawrence contributed to this report.