WASHINGTON -- A lukewarm endorsement from Defense Secretary Robert Gates and opposition among some lawmakers cast doubt Tuesday on whether Congress this week would lift a 17-year-old ban on gays serving openly in the military.

Gay rights' groups predicted that the bill might pass the House but 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 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

Gates has said that 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 Barack 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 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., and 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 on 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.

While Webb said he would oppose the measure, Sen. Susan Collins, a Republican, announced Tuesday that she would support it. Sen. Ben Nelson, a conservative Nebraska Democrat who sits on the panel, declined to say how he would vote.

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 would prevent the issue from becoming the subject of midterm elections.

Pentagon press secretary Geoff Morrell said that Gates "continues to believe that ideally the DOD review should be completed before there is any legislation to repeal the 'don't ask, don't tell' law."

"With Congress having indicated that is not possible, the secretary can accept the language in the proposed amendment," Morrell said.