WASHINGTON, D.C. -- Advocates of a bill that would overturn the military's "don't ask, don't tell" policy said their fight for repeal this year is far from over despite failing to pass the Senate with only days left in the lame-duck session. Senate Republicans on Thursday blocked the legislation, which would have lifted the military's 17-year-old ban on openly gay troops. The measure was tucked into a broader defense policy bill and had passed the House last spring. It failed in a 57-40 test vote, falling three votes short of the 60 needed to advance. GOP senators mostly united in defeating the measure on procedural grounds, insisting that the Senate vote on tax cuts first. Maine Sen. Susan Collins was the only Republican to support moving to debate the bill. Collins and Sen. Joe Lieberman are now pushing standalone legislation they insist could be considered before the Senate's target adjournment next week. Its prospects are uncertain, although Reid indicated he was open to bringing it up before the holiday break. If passed, the bill still would require House approval, with time growing short. "We've got at least 60 votes, so we're going to keep up the fight," said Lieberman, an independent from Connecticut. "But we're not kidding ourselves, this is not going to be easy." House Speaker Nancy Pelosi promised to do what she could to get repeal legislation on the president's desk by the end of the year. "An army of allies stands ready in the House to pass a standalone repeal of the discriminatory policy once the Senate acts," she said in a statement issued Thursday. The White House on Friday appeared to embrace that approach. "The president remains committed to seeing this repeal done before Congress leaves town this year," said press secretary Robert Gibbs. "And I think there could be legislative vehicles that start in the House as a stand-alone that can withstand procedural hurdles and put the Senate on the record on an up-or-down vote that would repeal 'don't ask, don't tell."' One gay rights group, the Servicemembers Legal Defense Network, scheduled a rally Friday on Capitol Hill to show Congress it wouldn't give up. "We must show our rage for repeal and insist the Senate stay in Washington until they have finished the job," said Aubrey Sarvis, the group's executive director. If the repeal push in Congress fails, gay rights advocates say they will shift their focus back to the White House. They say the Obama administration should drop its challenge of a California federal court ruling that the ban was unconstitutional. The Justice Department is appealing the ruling because it says the matter should be decided by Congress and not the courts. "There is no legal or military rationale for the current law, only prejudice." said Christopher Neff of the Palm Center, an advocacy group based in California. "It is now up to our civil leaders to consider every available legislative, executive and judicial option to move beyond 'don't ask, don't tell."' Secretary of Defense Robert Gates, meanwhile, expressed frustration Friday that Congress failed to act this week to repeal the law. But he said military should not prepare for a possible repeal until Congress takes action - even though the law is under assault in the courts. While the Pentagon has a plan to implement the repeal, any training or preparation for the change would confuse the troops, Gates said. Speaking as he returned from Afghanistan aboard a military plane, Gates said he was "disappointed but not surprised" by the Senate's action Thursday. If Congress is unable to pass repeal, he said, then "my greatest worry will be that then we will be at the mercy of the courts and all of the lack of predictability that that entails." A Pentagon study unveiled last week found two-thirds of troops thought repealing the gay ban would have little affect on their unit's ability to fight. But the military services' top uniformed leaders cautioned against overturning the policy too soon. Several told lawmakers that allowing gays to serve openly during wartime could meet resistance from combat troops, and would be divisive and difficult.