WASHINGTON, Sept. 21 (Xinhua) -- U.S. Senate on Tuesday blocked the repeal of a military ban on gays in a procedural vote, a major upset for gay rights proponents.

In the procedural vote, the Senate blocked debate on the repeal of "Don't Ask, Don't Tell" policy with 56 yes and 43 no. It takes 60 yes for debate to move ahead.

The ban prohibits homosexuals to serve openly in the military, and is a major contention issue in U.S. politics. The defeat means the repeal might be delayed indefinitely. With Democrats expected to lose seats during the mid-term elections in November, gay rights proponents see this vote as the last chance in a while to change the Clinton-era rule.

The repeal is attached to a 726 billion defense budget authorization bill. Senate Democrats attached the provision to the bill, hoping Republicans would not vote against it, but Republicans opposed it anyway.

The White House lent its weight in the debate. It issued a statement of administration policy earlier in the day through the Office of Management and Budget, saying it supports the provision in the defense budget bill that would repeal the ban.

The White House said it supports the provision "as it would allow for completion of the comprehensive review, enable the Department of Defense to assess the results of the review, and ensure that the implementation of the repeal is consistent with the standards of military readiness, effectiveness, unit cohesion, recruiting and retention."

U.S. President Barack Obama had made a campaign pledge to repeal the rule. Under the provision in the 2011 defense authorization bill, the repeal will not be implemented until the Pentagon finishes its review of how it would impact the military.

White House spokesman Robert Gibbs said the administration is " disappointed at not being able to proceed on the legislation," but he said they'll keep trying.