Washington ? The Senate agreed Tuesday to end a 10-year-old ban on research and development of low-yield nuclear weapons, rejecting Democratic claims that lifting the prohibition would be a step toward nuclear war.

A Democratic amendment to keep the ban was defeated in a 51-43 procedural vote. After the vote, Democrats were offering a compromise that would allow research on the weapons to begin, but prohibit their development.

The Bush administration’s request to lift the research and development ban is part of a bill authorizing $400.5 billion in 2004 defense programs. A vote on the overall bill is expected this week.

Low-yield weapons have a blast equivalent to less than 5 kilotons, about a third as large as the bomb dropped on Hiroshima in World War II.

Democrats said lifting the ban would undermine U.S. efforts to persuade other countries not to develop their own nuclear weapons. They also said it would blur the line between nuclear and conventional weapons and make it more likely that a nuclear weapon might be used.

“This issue is as clear as any issue ever gets. You’re either for nuclear war or you’re not. Either you want to make it easier to start using nuclear weapons or you don’t,” said Sen. Ted Kennedy, D-Mass.

“Is half a Hiroshima OK? Is a quarter Hiroshima OK? Is a little mushroom cloud OK? That’s absurd. The issue is too important. If we build it, we’ll use it,” he said.

Advocates of lifting the ban said weapons could target enemies more precisely while limiting deaths of civilians.

They said such a weapon could be used to destroy stockpiles of chemical and biological weapons and avoid the kind of widespread contamination that might follow a strike using conventional weapons with less destructive force.

“To stop research and development on a potential weapon that could destroy a terrorist group or a rogue nation from creating a chemical/biological capacity that’s deep underground is illogical,” said Sen. Lindsey Graham, R-S.C.

Defense Secretary Donald Rumsfeld, at a news conference Tuesday, stressed that the Pentagon wanted only to consider the weapons.

“It is a study. It is nothing more and nothing less,” he said. “And it is not pursuing. And it is not developing. It is not building. It is not manufacturing. And it’s not deploying. And it is not using.”

The House version of the defense authorization bill also would remove the ban on research, but not development, of low-yield weapons. House debate on the bill is expected to begin today.