US President faces the challenge of convincing Americans he has clear military aims and a U.S. exit strategy in the Libya conflict as he seeks to counter growing congressional criticism

US President Barack Obama faces the challenge on Monday of convincing Americans he has clear military aims and a U.S. exit strategy in the Libya conflict as he seeks to counter growing congressional criticism.

In a high-stakes televised address, Obama -- accused by many lawmakers of failing to explain the U.S. role in the Western air campaign against Libya's Muammar Gaddafi -- will try to define the mission's purpose and scope.

His task was made easier when NATO agreed on Sunday to assume full responsibility for military operations in Libya, ending uncertainty about who would lead the allied effort.

Obama is expected to hail the alliance's decision as proof he is making good on his pledge that the United States -- with its forces entangled in Iraq and Afghanistan -- will play only a limited role in a war in a third Muslim country.

But Obama still must reassure an American public preoccupied with domestic economic concerns that intervention in Libya serves U.S. national interests and also overcome doubts that he has a clear idea of an end game.

"I know how concerned people are, and obviously the president will speak to the country Monday night to answer a lot of those concerns," U.S. Secretary of State Hillary Clinton told CBS News' "Face the Nation."

Republicans have been the most outspoken in their complaints that he has failed to communicate thoroughly the mission's goals, and some have chided him for not seeking congressional approval. While most fellow Democrats are still backing him, some see a lack of a coherent exit plan.

What remains unclear, however, is what happens if Gaddafi stays in power despite a no-fly zone and air strikes.

Obama has yet to address that scenario -- aside from reiterating that U.S. ground forces would not be used -- and it was not known how far he would go in his speech at the military's National Defense University in Washington.