Under the War Powers Resolution, the president must get authorization from Congress to continue a military mission.
But there are no signs he plans to do it for the current U.S. action in Libya. The White House said he doesn't need congressional approval because there are no ground forces in Libya and the U.S. does not face hostilities.
But even some leading Democrats disagree.
"I think the War Powers Act and Constitution make it clear that hostilities by remote control are still hostilities," Sen. Richard J. Durbin, D-Ill., said.
But Defense Secretary Robert Gates believes the president has complied with the law.
"I was in the White House and the NSC staff not long after the War Powers Act was passed," Gates said. "And I believe that President Obama has complied with the law, consistent in a manner with virtually all of his predecessors.
"I don't think he's breaking any new ground here," he said.
Republican Sen. John McCain is expected to offer a resolution in support of the mission, but other legislators have suggested cutting funding.