SHOWDOWN ON DADT SET FOR TUESDAY AFTERNOON…. The fate of “Don’t Ask, Don’t Tell” will hinge on the events of Tuesday afternoon. Those who want to see the end of this indefensible policy — especially those in Maine and Massachusetts who want to see the law changed — are going to have to pick the phone over the next 72 hours.

Up until fairly recently, repeal looked to be on track. A huge majority of Americans support scrapping the existing policy, as do President Obama, a majority of the House, a majority of the Senate, the Secretary of Defense, the chairman of the Joint Chiefs of Staff, and two former Joint Chiefs leaders, including Colin Powell.

The repeal provision is already included in the 2011 defense authorization bill, which, among other things, funds the U.S. military in the midst of two wars. The Senate Democratic leadership announced yesterday that it would bring the spending bill to the floor at 2:15pm (ET) on Tuesday — where Senate Republicans, led by a tantrum-throwing John McCain (R-Ariz.), may unanimously prevent lawmakers from voting.

Gay rights advocates went on high alert Thursday afternoon after Sen. Joe Lieberman (I-Conn.), a key proponent of repealing the military’s “don’t ask, don’t tell” policy, said that he believes Senate Republicans can kill the defense authorization bill containing the repeal — a step Sen. John McCain has already threatened to take.

“The question is whether the Senate leadership can negotiate an agreement with the Republicans that will allow the bill to come up and get them to feel that they can introduce amendments that they want to introduce as well,” Lieberman said in an interview with Kerry Eleveld of the Advocate, a web site and magazine for the gay community. “But until that happens, I don’t think the votes are there to break the filibuster, which would be a shame.”

McCain (R-Ariz.), the ranking member of the Senate Armed Services Committee, declared he will block the defense bill if the “don’t ask, don’t tell” language isn’t stripped.

Just to be clear, Lieberman is actually really good on this issue, and will fight in support of passage. McCain, meanwhile, has already won his primary, and his re-election appears assured — but he really hates gay people, even those who volunteer to put their lives on the line for America.

Sen. Susan Collins (R) of Maine voted with Democrats at the committee level, and it looked like she would help overcome a Republican filibuster on DADT. Now, however, Collins has slipped backwards, and is no longer committed to doing the right thing. What’s more, because Sen. Scott Brown (R-Mass.) had said he would not support a filibuster on this, but he’s prepared to go back on his commitment, too.

If Republicans stick together, they can prevent a vote and kill DADT repeal. That means the existing law — the one opposed by the public, the president, a majority of Congress, and the Pentagon — would remain in place for at least another two years, after expected Republican gains in the midterms.

Tuesday afternoon, in other words, is a do-or-die showdown. Those who care — about basic fairness, about ending discrimination, about military readiness, about national security, about saving taxpayer money, about the careers of dedicated servicemen and women who are needlessly being thrown out of the military during two wars — are going to have to pick up the phone and call the Senate.

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.