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.

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.