WASHINGTON — Maine Sen. Susan Collins, a Republican considered a critical vote on the issue, says she will support legislation that would repeal the military’s “don’t ask, don’t tell” policy.

Collins is a member of the Senate Armed Services Committee, which was expected to vote Thursday on whether to include the repeal provision in the 2011 defense authorization bill.

Some conservative Democrats on the panel have been reluctant to get behind the measure, saying they want to give the Pentagon more time to study the policy change.

Collins’ support comes amid doubts on whether there will be enough votes in Congress to lift the 17-year-old ban on gays serving openly in the military. Defense Secretary Robert Gates has offered lukewarm support for the repeal, and some lawmakers are opposing it, at least for now.

Gay-rights’ groups predicted that the repeal might pass the House, but will face a tough road in the Senate.

“The door isn’t closed, but it’s barely cracked,” said Aubrey Sarvis, executive director of Servicemembers Legal Defense Network.

A compromise was struck Monday by the White House and a small group of Democrats who fear that repeal efforts will be doomed if Republicans regain control of one or both houses of Congress after the fall elections.

The plan would overturn the “don’t ask, don’t tell” law but still allow the military to decide when and how to implement any changes to accommodate the new policy.

Gates has said he supports repeal but would prefer that Congress wait to vote until he can talk to the troops and chart a path forward. A study ordered by Gates is due Dec. 1.

Some lawmakers took a similar stand. “I see no reason for the political process to pre-empt it,” Sen. Jim Webb, a conservative Democrat from Virginia, said of the military study.

On Tuesday, Gates said he would support the White House compromise but wished it didn’t have to happen now.

President Obama has vowed to help repeal the 1993 law, which prohibits the military from asking service members whether they are gay, bans homosexual activity and requires that gay troops not discuss their sexual orientation.

Gates and Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, say they agree that the ban should be lifted but want time to complete a wide-ranging study on how to do so without causing turmoil.

With the political clock ticking, several lawmakers were planning this week to push for an immediate suspension on military firings related to sexual orientation.

In a deal brokered by the White House on Monday, Sens. Carl Levin, D-Mich., Joe Lieberman, I-Conn., and Rep. Patrick Murphy, D-Pa., announced they would introduce repeal legislation that would require military approval before it would take effect.

“They say they favor repeal,” Levin said of the administration on Tuesday. “There’s no reason why (Congress) should not have that same kind of expression.”

Added Murphy, an Iraq war veteran: “We need to get this done, and we need to get it done now. … We are moving forward.”

The House was expected to vote as early as Thursday on the measure as an amendment to the 2011 defense authorization bill.

Also Thursday, the Senate Armed Services Committee was to decide whether to include the provision in its version of the defense authorization bill. Tucking the repeal law into a broader defense bill authorizing the spending of hundreds of billions of dollars for U.S. troops would significantly strengthen the provision’s chances when it comes to the Senate floor for debate.

Levin said he wasn’t sure he had enough support to pass the measure, as speculation surfaced that the committee vote would fall at least one short.

Webb said he would oppose the measure. Sen. Ben Nelson, a conservative Nebraska Democrat who also sits on the panel, declined to say how he would vote.

Sen. Scott Brown, a Republican seen by many Democrats as a potential swing vote because he represents the heavily Democratic state of Massachusetts, said he was unlikely to agree to a repeal this week.

“I am keeping an open mind, but I do not support moving ahead until I am able to finish my review, the Pentagon completes its study, and we can be assured that a new policy can be implemented without jeopardizing the mission of our military,” Brown said.

On the House side, Murphy said he was confident the measure would pass despite opposition from leading Democratic Rep. Ike Skelton of Missouri.

Skelton, chairman of the House Armed Services Committee, said Tuesday he supports the ban for now and hopes his colleagues would “avoid jumping the gun.”

In addition to giving Gates time to study the matter, delaying a vote until after the elections would prevent the issue from becoming the subject of midterm elections.