Washington (CNN) -- Failing to repeal the law prohibiting openly gay and lesbian people from serving in the military leaves the services vulnerable to the possibility the courts will order an immediate and likely chaotic end to the policy, Defense Secretary Robert Gates told reporters Friday.
Gates, speaking aboard an aircraft as he traveled in the Middle East, said that "my greatest worry will be that we are at the mercy of the courts and all of the lack of predictability that that entails."
The Senate on Thursday rejected a Democratic bid to open debate on a defense authorization bill that includes a repeal of the "don't ask, don't tell" policy. The House has already passed the repeal measure, but with time running out in the current lame-duck session of Congress, Democrats were uncertain they could overcome Republican opposition and approve the proposal.
Democrats were pushing for action now because the new Congress in January brings a Republican-controlled House and a diminished Democratic majority in the Senate, which will make repealing "don't ask, don't tell" more difficult.
On Friday, about 100 people gathered near the U.S. Capitol to urge legislators to pass the repeal. One of them, retired U.S. Army Maj. Gen. Dennis Laich, said Republican opponents of the repeal measure were "absent without leave" in their legislative responsibility, while the Servicemembers Legal Defense Network called for the Senate to put off its holiday recess until "the task is finished."
The best hope for the demonstrators might be a bipartisan proposal announced Thursday that separates the repeal provision from the larger defense authorization bill. Sen. Joe Lieberman, an independent from Connecticut who caucuses with the Democrats, said the separate repeal measure had a chance to win the support of four Republican senators to overcome a GOP filibuster that blocked the issue on Thursday.
Gates said he was "disappointed in the Senate vote, but not surprised."
During a Senate hearing on December 2, Gates expressed his support for repealing "don't ask, don't tell," saying it could be done without a serious risk to military readiness if there was enough time for an orderly transition.
The military already has experienced the confusion Gates predicted if the courts, rather than Congress, decide the fate of the policy.
In October, military commanders instructed recruiters to begin processing openly gay and lesbian applicants for enlistment after a federal judge in California ruled the policy was unconstitutional and ordered an immediate end to the ban.
The military's order to recruiters was soon rescinded when a three-judge panel of the 9th U.S. Circuit Court of Appeals issued a stay of the judge's order at the request of the Obama administration. The case remains under appeal.
A military working group that studied the repeal issue has made some recommendations about implementing the policy, but more time is needed to respond to the myriad challenges and questions posed by allowing openly gay and lesbian service members, Gates said Friday.
GOP senators have pledged to block Senate business until after approval of an extension of Bush-era tax cuts and action on a bill authorizing government spending for the fiscal year, forcing Democrats to seek a deal with at least two Republican senators to end the filibuster. The Democratic caucus has 58 members. Senate rules require 60 votes to end filibusters.
In Thursday's vote, one Republican senator, Susan Collins of Maine, sided with the Democrats. Newly elected Democratic Sen. Joe Manchin of West Virginia voted with Republicans, killing any chance for the Democrats to succeed.
President Barack Obama said in a statement after the vote that he was "extremely disappointed."
Noting support for repeal from the defense secretary, the Joint Chiefs of Staff chairman and a majority of Americans, Obama said the "don't ask, don't tell" policy "weakens our national security, diminishes our military readiness, and violates fundamental American principles of fairness, integrity and equality."
"While today's vote was disappointing, it must not be the end of our efforts," Obama said. "I urge the Senate to revisit these important issues during the lame-duck session."
Gay rights advocacy groups, including those comprising military personnel, immediately condemned the Senate vote.
"Today leaders of both parties let down the U.S. military and the American people," said Joe Solmonese, president of the Human Rights Campaign. "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."
CNN's Paul Courson, Tom Cohen and CNN's senior congressional correspondent Dana Bash contributed to this report.