WASHINGTON — President Barack Obama on Monday declared the U.S.-led military intervention in Libya a success, saying it averted a massacre by longtime leader Moammar Gadhafi and that NATO’s takeover of the multilateral mission this week means the U.S. can quickly shift to a support role with less risk and cost.

“”Tonight, I can report that we have stopped Gadhafi’s deadly advance,”” Obama said in a speech at National Defense University in Washington. The address was designed to respond to criticism that he hasn’t explained the goals for U.S. involvement sufficiently. “”The United States of America has done what we said we would do.””

In the 27-minute speech, Obama made two parallel cases: first, that doing nothing would have run counter to U.S. ideals and national interests; and second, that to have acted alone or expanded the military mission to topple Gadhafi would have been too costly and repeated the mistakes of the Iraq War.

Obama said that America had a moral imperative in preventing Gadhafi from inflicting “”a massacre”” on his own people.

Comparing Libya’s largest rebel-held city to the U.S. city that will host the 2012 Democratic National Convention, Obama said that “”if we waited one more day, Benghazi, a city nearly the size of Charlotte, could suffer a massacre that would have reverberated across the region and stained the conscience of the world.

“”It was not in our national interest to let that happen,”” he said. “”I refused to let that happen.””

He said there also was a strategic U.S. interest in blocking the Libyan leader. Otherwise the fragile democracy movements in Tunisia, Egypt and across the Arab world would be endangered, as tyrants would draw the lesson that “”violence is the best strategy to cling to power.””

“”It is true that America cannot use our military wherever repression occurs,”” Obama said. “”But that cannot be an argument for never acting on behalf of what’s right.””

Still, the president didn’t explain why that logic doesn’t require intervention against tyrannical repression recently employed in Syria, Bahrain and Yemen, except to say that there were unique circumstances in Libya.

“”In this particular country — Libya,”” Obama said, “”we had a unique ability to stop that violence: An international mandate for action, a broad coalition to join us, the support of Arab countries, and a plea for help from the Libyan people themselves. We also had the ability to stop Gadhafi’s forces in their tracks without putting American troops on the ground.””

The timing and staging for the president’s remarks — 7:30 p.m. on a Monday evening, off-campus from the White House — seemed designed to deflate any impression that the Libyan intervention represents a bold new turn in U.S. foreign policy. Obama took care to define the Libyan mission as limited, with the U.S. role only contributing — and shrinking — to a singular multilateral mission led by others.

Obama aimed to clarify his goals for two specific audiences: average Americans, and insider elites including Congress, the military and foreign governments. Obama’s speech came on the eve of an international conference in London to discuss Libya’s future.

About a half hour before Obama spoke, White House press secretary Jay Carney issued a statement saying that the president and leaders of France, Germany and Great Britain had spoken by videoconference and agreed that Gadhafi “”should leave power”” and that “”the Libyan people should have the political space to determine their own future.””

A poll released Monday by the nonpartisan Pew Research Center underscored the U.S. public’s lack of clarity about the mission — fully 50 percent of Americans said U.S. and allied goals aren’t sufficiently clear. But more Americans favor U.S. involvement in Libyan airstrikes than oppose it, 47 percent to 36 percent, according to the survey, conducted March 24-27.

Perhaps equally telling, however, is that most Americans aren’t all that focused on Libya. In a month overloaded with the Japanese natural disasters and nuclear crisis, multiple Middle East revolutions and the March Madness college-basketball playoffs, only 15 percent said Libya was the news event they’re following closest.

“”There’s no sense the public is turning away from it in a big way,”” said Pew associate director Carroll Doherty. “”They’re just expressing doubts about the goals and aims of it.””

Congress and media commentators, however, are more critical, and were a primary audience that Obama was trying to calm. He has drawn criticism from lawmakers on both sides of the aisle who feel he didn’t consult them sufficiently before joining the intervention, and from media commentators of every ideological stripe who have found his mission rationale confusing and unsettling.

The president’s speech began a battery of administration efforts to defuse such questions. Defense Secretary Robert Gates, Secretary of State Hillary Clinton and other officials are to give a classified briefing Wednesday to lawmakers. And at least three congressional committees are scheduled to hold public hearings on Libya on Thursday.

Sen. John McCain, R-Ariz., the top Republican on the Senate Armed Services Committee, reacted with a mix of praise and criticism to Obama’s remarks.

“”I welcome the president’s clarity that the U.S. goal is for Gadhafi to leave power,”” McCain said in a statement. “”But an equal amount of clarity is still required on how we will accomplish that goal.””

Some Republicans were just critical. Rep. Tom Price of Georgia, the chairman of the House Republican Policy Committee, said NATO’s new lead role offers “”no assurance that American military men and women as well as American resources will not continue to play a very large part in the days to come.””

Rep. Bruce Braley, R-Iowa, was disappointed that Obama didn’t spell out what the Libya mission is costing taxpayers. “”We’ve got two wars in Iraq and Afghanistan — and Americans deserve to hear from our president what this third conflict is going to cost us.””

Several Democrats lauded Obama’s speech. Senate Majority Leader Harry Reid, D-Nev., stood firmly behind the president.

But in the House of Representatives, where many liberal Democrats are uncomfortable with the intervention, House Minority Leader Nancy Pelosi, D-Calif., chose her words carefully. She said she salutes the U.S. military but emphasized that “”U.S. actions in Libya will be strengthened by continued consultation with Congress.””