Senate Republicans blocked a major year-end push by Democrats to lift the military's ban on openly gay troops on Thursday, dealing a huge blow to gay rights groups' hopes for repeal of "don't ask, don't tell" any time soon. President Barack Obama instantly appealed to lawmakers to make another, last-ditch try before going home for the year.

The day's dramatic events left the fate of the issue in limbo, with lawmakers eager to adjourn and still facing numerous other contentious issues.

The Senate's 57-40 vote fell three short of the 60 needed to overcome procedural hurdles to lift the 17-year-old ban. Sen. Susan Collins of Maine was the lone Republican voting to advance the bill, and Sen. Joe Manchin of West Virginia was the only Democrat to vote against it.

The rejection was a defeat for Obama, who campaigned promising to overturn the law and later called it one of his top legislative priorities for the year. But in recent weeks the White House has done little to push the legislation, focusing its influence instead on tax cuts and a nuclear arms treaty with Russia.

Obama wasn't giving up. He said the ban "weakens our national security, diminishes our military readiness and violates fundamental American principles of fairness, integrity and equality." And he said repeal is supported by the military and the American people.

"I urge the Senate to revisit these important issues during the lame duck session," he said.

Senate Majority Leader Harry Reid was biting in his comments about Republican foes. "The other side may feel passionately that our military should sanction discrimination based on sexual orientation, but they are clearly in the minority," he said. "And they have run out of excuses."

But Republicans faulted him for the way the issue was brought to a vote, saying the procedure sealed the outcome.

The 1993 law bans gay troops from publicly acknowledging their sexual orientation. A repeal provision was included in a broader defense policy bill and passed last spring in the House.

More than 60 senators were expected to support repeal of the ban, including at least four Republicans. But GOP senators were united in demanding that the chamber vote on tax cuts first. They also wanted assurances by Reid they would be given extensive time to debate the defense bill, which contained other divisive provisions including one that would allow abortions at overseas military facilities.

Two senators, Republican Collins and independent Joe Lieberman of Connecticut, said they now would introduce a stand-alone measure to repeal "don't ask, don't tell." Its prospects are uncertain, though Reid indicated he was open to bringing it up before adjournment. If passed, it still would require House approval with time growing short.

Gay rights advocates were furious about Thursday's events.

"Instead of doing what is right, the world's greatest deliberative body devolved into shameful schoolyard spats that put petty partisan politics above the needs of our women and men in uniform," said Joe Solmonese, president of the Human Rights Campaign.

Democrats had said Thursday morning they remained hopeful a last-minute deal could be struck with Collins, believing her support would persuade other GOP senators — namely Massachusetts Sen. Scott Brown and Alaska Sen. Lisa Murkowski — to advance the legislation.

In the end, however, Reid said time was running out and called for a vote before a deal could be struck.

Collins said she didn't understand why Reid wasn't doing more to accommodate GOP concerns.

"There was such a clear path for us to be able to get this done," she said on the Senate floor. "I'm perplexed and frustrated that this important bill will become victim of politics."

Republican Olympia Snowe of Maine said the repeal "should have been considered apart from the comprehensive defense authorization bill, respecting the sensitive nature of the issue and providing ample time for floor debate." She said that "any opportunity to move forward with this discussion has been undermined by the majority's desire to score political points in the remaining days of this legislative year."

The Senate vote came after former Democratic Sen. Sam Nunn announced in an interview with The Associated Press that he thinks gays could serve openly without damaging the armed forces' ability to fight. Nunn, who had led opposition to gays in the military in 1993, said he would advise that the Pentagon be given at least a year to prepare troops for the change.

"Society has changed, and the military has changed," the former senator from Georgia said.

Last week, the Pentagon unveiled a study that found two-thirds of troops thought repealing the ban would have little impact on their units' ability to fight.

Still, the service's top uniformed leaders cautioned about overturning the policy too soon.

In congressional testimony last week, three of the four service chiefs said they would oppose lifting the ban during wartime because of resistance among combat troops.

While most troops signaled they didn't care if gays served openly, nearly 60 percent of the Marine Corps and Army soldiers in combat arms units predicted problems would arise.

Manchin said he voted against the bill because repeal shouldn't happen while troops are fighting in Afghanistan.

"I think it's going to happen and probably should happen as far as our repealing it," he said. But "it's a timeliness issue with me. There's a war going on."


Associated Press writer Andrew Taylor contributed to this report.