WASHINGTON (Reuters) - President Barack Obama is expected to talk about the "Don't ask, don't tell" rule that bars openly gay Americans from serving in the military during his State of the Union address this week, a senior lawmaker said on Monday.

During his 2008 campaign for the presidency, Obama vowed to end the rule discriminating against gays and he renewed that pledge in a speech last year.

"We were told by the Pentagon that they expected the president to say something in the State of the Union on it," said Senator Carl Levin, chairman of the Senate Armed Services Committee, referring to the speech Obama will deliver on Wednesday evening to a joint session of the U.S. Congress.

But Levin added: "I have no idea" what Obama will say.

The rule, which dates to the presidency of Bill Clinton, requires homosexuals to keep quiet about their true sexual orientation if they want to be in the U.S. military. It also stops recruiters or commanders from asking members of the armed forces whether they are gay.

It was a compromise signed into law by Clinton in 1993 after the military objected to his calls for welcoming openly gay Americans into their ranks.

Levin, a Democrat, is among members of Congress who favor ending the policy and allowing gays to serve openly in military ranks.

Levin plans to hold a hearing of his committee on the policy by early February, he told reporters on Capitol Hill. He had considered holding the hearing in late January, but the Obama administration asked him to hold off until after the State of the Union speech.

In his speech at 9 p.m. on Wednesday, Obama will lay out the challenges and set the tone for his administration in 2010.

Critics charge that having gays openly serve in the military would undermine morale and discipline. Any such proposal from Obama is likely to be controversial.

Some key lawmakers oppose repealing the ban, including House of Representatives Armed Services Committee Chairman Ike Skelton, Levin's counterpart. Gay rights advocates have accused Obama of dragging his feet about keeping a campaign pledge.