President Bush, proposing an orderly, long-term reduction in the nation's military arsenal, is backing a 25 percent cut in the armed forces by the mid-1990s, administration and Pentagon sources say. Bush was slated to make the proposal in a speech today in Aspen, Colo., as his $307 billion defense budget for fiscal 1991 has come under severe attack on Capitol Hill. ``The president will lay out the broad outlines of his new strategy for the nation's defenses and how our military must respond, based on the changes in the world that have occurred,'' said a senior administration official, who spoke on condition of anonymity. In a speech to the Aspen Institute, Bush planned to offer support for two hotly debated weapons programs _ the B-2 stealth bomber and the Strategic Defense Initiative _ and support a 25 percent cut in the armed forces by the end of 1995, the source said. A similar cut in the nation's arsenal of troops and weapons was laid out by Defense Secretary Dick Cheney as an option for budget negotiators in June, but it was derided as ``overly cautious'' by critics seeking additional budget savings. Bush planned to make the address after conferring with British Prime Minister Margaret Thatcher in Aspen. It will be the fourth time Bush and Mrs. Thatcher have met in as many months. Bush's speech comes two days after the House Armed Services Committee cut $24 billion from the $307 billion the president had requested for defense spending in the fiscal year beginning Oct. 1. The panel voted to halt production of the B-2 bomber and slashed almost $2 billion from Bush's request of $4.7 billion from the anti-missile Strategic Defense Initiative, commonly known as Star Wars. It also cut military personnel by 129,500 and scaled back on two nuclear missile programs. In June, Cheney said a 25 percent force reduction could save $8.6 billion over five years as the Pentagon retired six active duty Army divisions, 111 Navy ships and 11 Air Force tactical fighter wings, and removed 442,000 men and women from the military's 2.1-million-member active duty ranks. Appearing Wednesday evening on public television's ``MacNeil-Lehrer NewsHour,'' the defense secretary hinted that even more changes could be in store. He said he had ``new base closings'' to announce, possibly as early as next week. But Cheney also complained that while the House wanted him to cut personnel, members were balking at closing unneeded bases. Bush said on July 20 at a Strategic Air Command base in Cheyenne, Wyo., that, ``We must have an orderly build-down, not some kind of a fire sale'' in defense. He indicated then that he could live with the $18 billion that the Senate Armed Services Committee voted to cut from his defense budget, but he implored Congress to ``hold the line at (that) already painful reduction.'' The senior American official said the president's remarks would follow along the lines of those made by Cheney and Gen. Colin Powell, the chairman of the Joint Chiefs of Staff. Both have called repeatedly for an orderly, planned reduction in troop strength so that the all-volunteer force is not gutted and morale destroyed. Powell has ordered a study to assess the ``base line force'' that the nation needs to be a superpower, another source said. Mrs. Thatcher is scheduled to address the closing session of the Aspen Institute conference on Sunday. Like Bush, she is under pressure at home to deliver a peace dividend reflecting the changing face of Europe and reduced East-West tensions. While the United States grapples with how to reduce the Pentagon budget, Mrs. Thatcher's defense secretary, Tom King, announced plans last week to reduce the size of the British army by 25 percent over five to seven years and cut its forces in West Germany in half. King also outlined cuts in the British air force and navy but said Britain would retain its nuclear arsenal. Cheney was flying separately to Aspen to attend the symposium at the non-profit institute, marking its 40th anniversary of bringing political, business and cultural leaders together to discuss global issues.