WASHINGTON (Reuters) - Senate Republicans on Thursday blocked a White House-backed bid to repeal the ban on gays serving openly in the U.S. military, but lawmakers seeking to end the policy said they would soon try again.

Veteran Evelyn Thomas is arrested after handcuffing herself to a fence at the White House in Washington November 15, 2010. Thomas and twelve others were arrested in their protest against the "Don't Ask, Don't Tell" law that prevents openly gay men and women from serving in the U.S. military. REUTERS/Kevin Lamarque

On a 57-40 vote, Democrats fell short of the needed 60 votes to clear a Republican procedural hurdle and move to end the 17-year-old policy known as “don’t ask, don’t tell.”

Just one Senate Democrat, Joe Manchin, voted to sustain the procedural roadblock while one Republican, Susan Collins, voted to end it.

Collins and Senator Joe Lieberman, an independent, said afterward they intend to offer a new bill to end the military policy and push for its passage once a stalemate over taxes is resolved.

“We’re not going to give up. We’ve got the votes to change this unjust policy,” Lieberman told reporters, saying that at least three more Senate Republicans may vote with them.

President Barack Obama urged Senate backers to keep pushing to end the policy. “While today’s vote was disappointing, it must not be the end of our efforts,” Obama said.

“I have pledged to repeal this discriminatory law,” Obama added, noting the most Americans support his position.

“This law weakens our national security, diminishes our military readiness, and violates fundamental American principles of fairness, integrity and equality,” Obama said.

People who oppose gays serving openly in the military argue that lifting the ban would undermine good order and discipline and harm unit cohesiveness.

Senate Republicans recently vowed to block any bill -- other than ones to fund the government -- until expiring tax cuts are extended for millions of Americans. Democrats and Republicans hope to reach a deal soon to do it.

DEFENSE POLICY BILL

The measure the Senate voted on Thursday was a sweeping defense policy bill, which contained the proposed repeal to the military policy on gays.

At least 13,000 men and women have been expelled from the military under “don’t ask, don’t tell,” which allows gay people to serve in the armed forces as long as they keep their sexual orientation secret. It was passed by Congress and implemented in 1993 under Democratic President Bill Clinton.

Obama, a Democrat, took office in January, 2009, vowing to end the policy, calling it unfair and unwise.

But Obama has faced opposition from Republicans led by his Republican adversary in the 2008 U.S. presidential election, Senator John McCain, a former Navy fighter pilot and prisoner of war in Vietnam.

At a hearing before the Senate Armed Services Committee last Thursday, McCain said it may be too early to end the ban and challenged a recent Pentagon study that forecast little impact if the policy were lifted.

Defense Secretary Robert Gates -- who asked “If not now, when?” in his testimony -- and Admiral Mike Mullen, the chairman of the military’s Joint Chiefs of Staff, advocate the repeal. But there is uneven support in the military.

Marine Corps Commandant James Amos, for example, has warned that ending “don’t ask, don’t tell” could be dangerous while the United States is fighting wars in Iraq and Afghanistan.

A recent Pentagon study found the repeal would likely have little impact but said a “significant minority” within the military expressed negative views or concerns about repeal, mostly among Marines or combat soldiers.

Time is running out for a legislative repeal this year.

This session of the Congress may end next week. When the new Congress convenes next month, Republicans will control the House of Representatives and have five more seats in the Senate, cutting the Democrats’ hold on the chamber to 53-47.

If Congress doesn’t end the policy, the federal courts, already involved in the fray, may ultimately decide the issue.

In California, U.S. District Judge Virginia Phillips ruled in September that the military’s ban infringes on U.S. free-speech and due-process rights.

Phillips put that opinion into effect on October 12 with a blanket injunction requiring the military to stop enforcing the ban and halt investigations and discharges stemming from it.

But the next week, a federal appeals court in San Francisco ruled that the Pentagon could reinstate the ban, pending further court action.