WASHINGTON (AP) — The military’s top uniformed officer on Tuesday made an impassioned plea for allowing gays to serve openly in uniform, telling a Senate panel it was a matter of integrity and that it is wrong to force people to “lie about who they are in order to defend their fellow citizens.”

The comments by Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, set the stage for the Defense Department’s yearlong study into how the ban can be repealed without causing a major upheaval in the military.

Defense Secretary Robert Gates, appearing with Mullen before the Armed Services Committee, announced plans to loosen enforcement rules involving the “don’t ask, don’t tell” policy that has been in effect since 1993.

President Barack Obama has called for a repeal of the policy, although he did little in his first year in office to advance that goal. If he succeeds, it would mark the biggest shake up to military personnel policies since President Harry S. Truman’s 1948 executive order integrating the services.

“No matter how I look at the issue,” Mullen said, “I cannot escape being troubled by the fact that we have in place a policy which forces young men and women to lie about who they are in order to defend their fellow citizens.” Noting that he was speaking for himself and not for the other service chiefs, Mullen added: “For me, it comes down to integrity — theirs as individuals and ours as an institution.”

Congress enacted the law that enshrined the “don’t ask” policy, which was intended to soften the previous blanket prohibition on gays in the military. “Don’t ask” says gays may serve so long as they kept their sexuality private. Gay rights organizations have called that an insult and have said it is a form of discrimination.

Repeal of the ban would require a new law passed by Congress. Gates and Mullen said their efforts are intended to make sure the Pentagon is ready when that time comes.

The hastily called session gave Obama high-level cover on a divisive social issue complicated by the strains on an all-volunteer military force of fighting two wars.

Gates, who says he is a Republican, is the only member of former President George W. Bush’s Cabinet whom Obama asked to stay on. He has gained a reputation for candor and caution. Mullen’s words were a forceful endorsement from a careful man, but his very appearance, starched uniform and four stars on view, spoke as loudly.

Gates drew unusually pointed and partisan criticism from Republicans on the panel for saying that the review will examine how, not whether, to repeal the ban.

Arizona Sen. John McCain, the top Republican on the committee, icily told Gates he was disappointed in his position. In sharp questioning, McCain angrily suggested that the Pentagon was usurping Congress’ job in rewriting the law should it choose to do so.

“Has this policy been ideal? No, it has not,” McCain said. “But it has been effective.”

Mullen looked pained when Sen. Jeff Sessions, R-Ala., suggested that Mullen had preordained the outcome of any study of the wisdom of repeal by signaling his own opposition to the ban.

“This is about leadership, and I take that very, very seriously,” Mullen replied, tightlipped.

Several other Republicans sided with McCain, warning Mullen and Gates not to pursue a change at a time when the United States is fighting two wars in Iraq and Afghanistan and facing a continuing threat of terrorism. Democrats said they would back a change in policy.

Sen. Carl Levin, D-Mich., and chairman of the committee, said a repeal of the law might be slipped into a broader military policy bill that authorizes defense spending.

Democratic Sen. Mark Udall said his Colorado constituents pride themselves on allowing others to live and let live.

“You don’t have to be straight to shoot straight,” said Udall, quoting libertarian Barry Goldwater.

Gates suggested that lawmakers keep the intensity of debate in check until the military can get a better handle on how to proceed. To sort out the details, Gates has turned to Pentagon counsel Jeh Johnson and Gen. Carter Ham, who leads Army forces in Europe.

“Keep the impact it will have on our forces firmly in mind,” the secretary implored lawmakers

Mullen said it was his sense that rank-and-file troops would support the change.

“I have served with homosexuals since 1968,” he said in response to questions from Sen. Jeff Sessions, R-Ala. “There are a number of things cumulatively that get me to this position.”

Ham is a former enlisted infantryman who rose through the ranks to eventually command troops in northern Iraq in 2004 and hold senior positions within the Joint Staff. Recently, he helped conduct an investigation into the shootings by a soldier at the Fort Hood Army base in Texas.

As the Pentagon’s top legal counsel, Johnson has played an integral role into the effort to try to close the military prison in Guantanamo Bay, Cuba.