By KEN GUGGENHEIM WASHINGTON (AP) - A Senate committee said Friday it had voted to lift a decade-old ban on the research and development of low-yield nuclear weapons, overriding Democratic arguments that repeal would damage U.S. efforts to stop the spread of nuclear arms."

This is a major shift in American policy," said Sen. Carl Levin of Michigan, the panel's top Democrat. "It just sort of makes a mockery of our argument around the world that other countries - India, Pakistan - should not test and North Korea and Iran should not obtain."But John Warner, R-Va., chairman of the Armed Services Committee, said, "Without committing to deployment, research on low-yield nuclear weapons is a prudent step to safeguard America from emerging threats and enemies."

The committee agreed to lift the ban as part of a bill authorizing $400.5 billion in 2004 defense activities. The measure was approved in closed session Thursday, and details were released Friday.

The total is slightly more than the amount requested by the Bush administration and about 4.7 percent more than was appropriated by Congress last year. The bill does not include the cost of the Iraq war, part of which was included in an $80 billion spending package approved last month.The committee's bill also would ease environmental restrictions that the Pentagon says limits its training, would authorize $9.1 billion for missile defense, and would increase military pay by an average 4.15 percent. It now goes to the full Senate.

The House Armed Services Committee is expected to complete its version of the bill next week. On Friday, its readiness subcommittees voted to cancel the 2005 round of base closings, which the Pentagon is sure to oppose.On Wednesday, the Armed Services Strategic Forces subcommittee this week had approved lifting the ban on low-yield weapons research.

Low-yield nuclear weapons have warheads of less than five kilotons, or about a third of the bomb dropped on Hiroshima in World War II. Combined with precision missiles, low-yield weapons could be used to hit a target without causing as much damage to surrounding areas as other nuclear weapons would.

Warner said the committee has been assured by Secretary of State Colin Powell that lifting the research and development ban would not affect nuclear proliferation."

America has had a ban on this research since 1993, yet that has done nothing to stop other countries from seeking to acquire nuclear weapons," Warner said.Opponents of the weapons question whether they are needed, given the force of the United States' conventional arsenal. Some fear they would make presidents less reluctant to use nuclear weapons in war.

Nuclear weapons have been "so onerous that you would not use them," said Sen. Jack Reed, D-R.I. "I think this departure, or this lowering of threshold of potential use, is a very dangerous trend."The Senate committee's bill does not authorize testing of low-yield weapons, but some Democrats say it would be merely a matter of time before tests would begin."

I think this opens the door that if you start developing new low-yield nuclear weapons, basically the next step after that is the testing of them," said Sen. Bill Nelson of Florida.The low-yield issue was one of two nuclear issues that divided Democrats and Republicans. On a party line vote, Republicans rejected a Levin amendment that would have required the Bush administration to get congressional approval before developing a nuclear earth penetrator.

The weapon would burrow into the earth and detonate, making it potentially useful against deep underground bunkers. The bill authorizes $15 billion to continue studying it.The environmental exemption in the bill is a compromise between the Pentagon, which wanted more sweeping changes to laws its says restricts military exercises, and environmental advocates who said exemptions were potentially damaging to wildlife.

The bill would exempt the military from the Endangered Species Act's requirement that it set aside undisturbed "critical habitat" important to the recovery of a rare animal or plant battling extinction.