Congress is close to ending the ban on gays serving openly in the military, with the Senate ready for a landmark vote that could deliver a major victory to the gay community, liberals and President Barack Obama.

Senators planned a procedural vote Saturday on a bill ending the Pentagon's "don't ask, don't tell" policy as lawmakers held an unusual weekend session in their race to finish the year's legislative business.

If at least 60 senators vote to advance the bill as expected, the repeal, which passed the House this week, could win final passage by late afternoon. Republicans opposed to changing the law could demand extended debate although early indications were that they might not bother.

With opposition from Republicans weakening, passage would mark a triumph for Obama, who made repeal of the 17-year-old law a campaign promise in 2008. It also would be a win for congressional Democrats who have struggled in the final hours of the lame-duck session to overcome Republican objections, and for gay rights groups who said Saturday's vote was their best shot at changing the law because a new GOP-dominated Congress will take control in January.

Advocates vowed to leave nothing to chance and stepped up lobbying efforts in the hours before the vote, including a silent protest in the visitor seats overlooking the Senate floor.

"We simply cannot let the clock run out and lose this historic opportunity," said Aubrey Sarvis, executive director of the Servicemembers Legal Defense Network, whose supporters vowed to sit in the Senate gallery until the law was repealed.

Repeal would mean that for the first time in U.S. history, gays would be openly accepted by the military and could acknowledge their sexual orientation without fear of being kicked out. More than 13,500 service members have been dismissed under the 1993 law.

Under the bill, the president and his top military advisers — the defense secretary and chairman of the Joint Chiefs of Staff — are required to certify to Congress that lifting the ban won't hurt troops' ability to fight. After that, 60 days must pass before any changes go into effect.

The House approved the bill earlier this week by a 250-174 vote.

A small but vocal group of Republicans led by Sen. John McCain of Arizona said the law shouldn't be changed during wartime.

"We send these young people into combat," said McCain. "We think they're mature enough to fight and die. I think they're mature enough to make a judgment on who they want to serve with and the impact on their battle effectiveness."

The Democratic push for repeal was strengthened by the release of a major Pentagon study that concluded gays could serve openly without affecting combat effectiveness. The assessment found that two-thirds of troops predicted little impact if the law is repealed.

The study was strongly backed by the Pentagon's top leadership, including Defense Secretary Robert Gates and Adm. Mike Mullen, chairman of the Joint Chiefs of Staff.

McCain has dismissed the study as flawed and cites concern among troops assigned to the front lines. Some personnel predicted openly gay troops would cause problems. Most of them were in combat arms units such as infantry and special operations.

The chiefs of the Army and Marine Corps warned Congress that repeal could pose serious problems if the law is overturned when troops are still fighting in Afghanistan.

Gen. James Amos, the head of the Marine Corps, has become the most outspoken opponent and claims letting gay troops serve openly could cost lives.

Gates and Mullen say this fear is overblown. They note the Pentagon's finding that 92 percent of troops who believe they have served with a gay person saw no impact on their units' morale or effectiveness.

The bill appeared all but dead earlier this month when Senate Republicans voted for a second time this year to block the measure. The language was tucked into a broader defense policy bill that many GOP senators said required more debate than Democrats would allow. They also objected to taking up any legislation before addressing tax cuts and government spending.

Senate Democrats addressed many of the procedural objections, including completing the tax-cut legislation. They also stripped the repeal provision from the defense policy bill.