Washington ? Senate Republicans on Tuesday blocked an effort by Democrats and the White House to lift the ban on gays from serving openly in the military, voting unanimously against advancing a major defense policy bill that included the provision.

Former U.S. Marine Morningstar Vancil, right, is hugged by Robert C. Potter after speaking with a reporter Tuesday at the Alexander Hamilton American Legion Post 448 office in San Francisco. According to its website, the Alexander Hamilton Post 448 is the only post whose membership is predominantly gay, lesbian, bisexual and transgendered.

The mostly partisan vote dealt a major blow to gay rights groups who saw the legislation as their best hope, at least in the short term, for repeal of the 17-year-old law known as “don’t ask, don’t tell.”

If Democrats lose seats in the upcoming congressional elections this fall, as many expect, repealing the ban could prove even more difficult — if not impossible — next year. With that scenario looming, Senate Majority Leader Harry Reid said that a lame-duck session was being planned and that lifting the ban would be taken up then.

The episode upset advocates who believe that neither President Barack Obama nor Reid did enough to see the measure through.

“The whole thing is a political train wreck,” said Richard Socarides, a White House adviser on gay rights during the Clinton administration.

Democrats included the repeal provision in a $726 billion defense policy bill, which authorizes a pay raise for the troops among other popular programs. In a deal brokered with the White House, the measure would have overturned the 1993 law banning openly gay service only after a Pentagon review and certification from the president that lifting the ban wouldn’t hurt troop morale.

But with little time left for debate before the November ballot, the bill languished on the Senate calendar until gay rights groups, backed by pop star Lady Gaga, began an aggressive push to turn it into an election issue.

Earlier this month a federal judge in Los Angeles declared the ban an unconstitutional violation of the due process and free speech rights of gays and lesbians. The decision was the third federal court ruling since July to assert that statutory limits on the rights of gays and lesbians were unconstitutional.

Reid agreed to force a vote on the bill this week and limit debate, despite Republican objections. A Nevada Democrat in a tight race of his own this fall, he also pledged to use the defense bill as a vehicle for an immigration proposal that would enable young people to qualify for U.S. citizenship if they joined the military.

Republicans alleged that Reid was using the defense bill to score political points with the Democratic base.

“This is not a serious exercise. It’s a show,” said Senate GOP leader Mitch McConnell of Kentucky.

Democrats countered that the bill merely reflects public opinion. Recent polls suggest that a majority of Americans think the ban on gays in the military should be overturned.

“We’re going to fight for this,” said Sen. Carl Levin, D-Mich., chairman of the Armed Services Committee.