Washington (CNN) -- Despite a high-profile push from pop star Lady Gaga and other gay rights supporters, 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. 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. 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.
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NEW: Collins lines up behind GOP
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A broad defense policy bill includes the repeal provision
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Republicans threaten a filibuster over limits on debate
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Reid includes non-defense measures in the bill
"Now is not the time to play politics, and I again call on the majority leader to work with @placeholder leaders to negotiate an agreement so that the Senate can debate the defense bill this week," Collins said.