* Democrats expected to try again in "lame-duck" session
* Republicans want to wait for Pentagon study
* Obama campaigned on promised to end military ban on gays (Adds analysts, polls, reaction)
By Susan Cornwell and Phil Stewart
WASHINGTON, Sept 21 (Reuters) - The U.S. Senate on Tuesday blocked a measure that would let homosexuals serve openly in the military, in a setback for one of President Barack Obama's policy goals.
Sixty votes were needed for debate to begin on the bill authorizing defense programs and repeal of the policy known as "don't ask, don't tell." Just 56 senators voted yes and 43 no, but the measure will probably have another chance this year.
Since 1993, homosexuals have been allowed to serve in the U.S. military as long as they hide their sexual orientation. They are expelled if it becomes known. Polls have said most Americans support lifting the ban, and repealing it was one of Obama's promises in his 2008 campaign.
Obama's Democrats in Congress wanted to scrap the "don't ask, don't tell" policy before November congressional elections in which Republicans are expected to make big gains, possibly making it more difficult to repeal the ban. The House has already voted to change the law.
Advocates of the change are expected to try again in the Senate during the "lame-duck" session of Congress that is expected to follow the elections. The White House said it supported another attempt.
"We're disappointed at not being able to proceed on the legislation, but we'll keep trying," White House spokesman Robert Gibbs said.
Every Republican present on Tuesday voted to block debate on the bill. Democrats Mark Pryor and Blanche Lincoln joined them.
It could be difficult squeezing the lengthy defense bill, which authorizes programs such as military pay increases and Pentagon weaponry, into the late-year lame duck session.
By the Pentagon by then may have finished a study of the "don't ask, don't tell policy. " Many Republicans, including Senator John McCain, said they opposed voting on a repeal of the policy before the Pentagon review is done.
Independent Senator Joe Lieberman, co-sponsor of the repeal, said he believed it could still pass. "We didn't win today, but we can win this fight this year."
Lieberman said 14,000 soldiers had been kicked out of the military because of the policy since 1993. That was "the equivalent of an entire division of war fighters, that we need in places like Afghanistan and elsewhere around the world."
'BLATANT POLITICAL PLOY'
Some Republicans were also unhappy with Democratic plans to bring up immigration-related legislation known as the "Dream Act" as an amendment to the bill before dealing with defense-related amendments. The "Dream Act" would provide a conditional path to citizenship for young illegal immigrants if they go to college or do military service.
McCain charged Democrats were aiming simply for Hispanic and gay votes. "This is a blatant political ploy in order to try to galvanize the political base of the other side which is faced with losing an election."
Democrat Carl Levin, the Armed Services Committee chairman, denied Congress was getting ahead of the military.
He said that even if Congress gave final approval to the bill, repeal of the ban would require certification from Obama, Defense Secretary Robert Gates and Admiral Mike Mullen, the top U.S. military officer. Levin said the repeal would not hurt readiness, unit cohesion, recruitment and retention.
The debate has become a rallying cry for gay rights activists. Singer Lady Gaga released a video last week urging senators and "fellow Americans" to push for repeal. Servicemembers United, an organization of gay and lesbian troops and veterans, expressed deep disappointment with Tuesday's vote.
Terry Madonna, a public affairs professor at Franklin and Marshall College in Lancaster, Pennsylvania, said Democrats could fit the vote into a narrative that Republicans were the party of "no" and out of touch with mainstream America.
But analysts questioned how powerful the issue would be in an election expected to be dominated by economic concerns. (Editing by Peter Cooney)
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