WASHINGTON - A lukewarm endorsement from Defense Secretary Robert M. Gates and opposition among some Democratic lawmakers cast doubt Tuesday on a White House-backed proposal to lift the ban on gays serving openly in the military.

Conservative Democrats were hesitant to back the compromise struck Monday by the White House and a small group of more liberal lawmakers 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 to accommodate the new policy.

Gates said Tuesday that he supported repeal but would prefer that Congress wait to vote until he could 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 preempt it," Sen. Jim Webb (D., Va.) said of the military study.

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 yet 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.) and Joseph 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 administration officials 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 bill's chances when it reaches the Senate floor for debate.