U.S. Defense Secretary Robert Gates is under pressure to rein in the Pentagon's budget after President Obama in April ordered $400 billion in defense cuts over the next 12 years.

Gates says defense savings on that scale are not possible without weakening U.S. military power.

The secretary will outline his budget proposals in an address to a think tank on Tuesday. His staff says it will be his final policy speech in Washington. He is set to step down from his cabinet post on June 30.

Gates successor, Leon Panetta, will be left with the tough task of meeting White House and congressional demands for new budget savings while the nation is still at war.

Gates says his goal is to preserve the military's fighting capability even while cutting its size.

"If the political leadership of this country decides that it must reduce the investment in defense by hundreds of billions of dollars, then I don't think we can afford to have anything that's off the table," Gates told reporters last week when asked whether the review would consider eliminating part of the U.S. nuclear arsenal.

Gates has said that all three "legs" of the U.S. strategy nuclear force - bombers, land-based missiles and nuclear-armed submarines - are due for expensive modernization.

"You may have to make some choices there," he said April 21, implying that one leg might have to be dropped.

Gates says he won't take an across-the-board approach -- making budget cuts evenly among the Army, Navy, Air Force and Marine Corps, for example. He calls that "managerial cowardice."

Instead, he wants to compel the White House and the Congress to decide what kind of military the country needs in the years ahead, then make budget-cutting decisions to fit that vision.