Republicans in the U.S. Senate have blocked an effort to repeal the law banning gays from serving openly in the military.

The partisan vote was a defeat for gay rights groups, which saw the provision in a defence authorization bill as their last chance for the foreseeable future to overturn the law known as "don't ask, don't tell."

Democrats fell short of the 60 votes needed to advance the legislation, which authorized $726 billion US in defence spending. The vote was 56-43 and fell mostly along party lines.

The vote was expected to be close amid campaigning for midterm congressional elections in November, which could see Republicans winning back majorities in the House of Representatives and possibly the Senate.

Some Democrats had reportedly expressed concern they could alienate conservative swing voters over the issue.

Recording artist Lady Gaga speaks at a rally in Portland, Me., on Monday in support of repealing the U.S. military's 'don't ask, don't tell' policy for gay service members. (Pat Wellenbach/Associated Press)

Republicans led by Senator John McCain of Arizona promised to continue delaying manoeuvres. They want to keep the law in place, at least until the Pentagon completes a survey later this year on repeal's likely effect on troops. The military is due to release the results on Dec. 1.

"We're in two wars. Shouldn't we assess the impact on the readiness, the morale and effectiveness of the men and women who are in harm's way," McCain said Monday.

Lady Gaga joins in the debate

Pop star Lady Gaga headlined a rally in Maine on Monday to pressure the state's two Republican senators, Olympia Snowe and Susan Collins, to join Democrats in overcoming filibuster attempts.

Snowe and Collins have previously voted with Democrats on social issues, but while they have expressed support for getting rid of the policy, each now appear undecided.

To loud cheers from the crowd, Gaga said she was proposing a new law titled, "If you don't like it, go home," which would remove from the military "the straight soldier who hates the gay soldier."

An estimated 13,000 people have been discharged under the law since its inception in 1993. Although most dismissals have resulted from gay service members outing themselves, gay rights groups say it has been used by vindictive co-workers to drum out troops who never made their sexuality an issue.

With files from The Associated Press