Democrats in the US Senate have said a move by the Bush administration to end a decade-long ban on researching "mini-nuke" would spark a new arms race.
The Senate voted 51-43 to uphold a request from President George W. Bush to repeal the ban on research and development of tactical nuclear weapons with one-third or less the force of the bomb dropped on Hiroshima in World War Two that killed more than 100,000 people.
Senators were to consider a compromise today that would allow research but bar engineering and development of the weapons as they continued work on a bill authorising $400.5 billion in defence programs for next year.
The defence bill the House of Representatives was to consider today already has similar compromise language on the warheads with a yield of less than five kilotons.
Without the prohibition on developing the low-yield weapons, Michigan Democratic Senator Carl Levin said: "It will appear to the world that this Senate is committed to the development of new nuclear weapons."
The Senate voted after defence Secretary Donald Rumsfeld pressed for a green light to study smaller nuclear weapons which he said may be useful in destroying deadly chemical and biological weapons stocks.
Mr Rumsfeld said the administration just wanted to study these weapons, "not to develop, not to deploy, not to use" them.
But Democrats said it was foolish to think the administration would invest in research on the weapons with no intention of producing them.
They also said Mr Bush's plan would encourage other countries to develop small nuclear weapons, reversing decades of US policy aimed at stemming the spread of nuclear weapons and preventing their use.
"The hardliners say things are different today. A nuclear war won't be so bad if you just make the nukes a little smaller," said Senator Edward Kennedy, a Massachusetts Democrat. "Is half of Hiroshima okay? Is a quarter of Hiroshima okay?"
But Alabama Republican Senator Jeff Sessions said it would be "irrational for us to prohibit research that could inform future decisions as to whether such weapons would enhance national security of our country."
Mr Rumsfeld and Air Force General Richard Myers, chairman of the Joint Chiefs of Staff, said the nuclear warheads may be useful to destroy deadly biological or chemical material that would be dispersed if struck by conventional weapons.
But Democrats said the low-yield arms had no known military use, and would produce devastating radioactive fallout.
Democrats also were set to challenge the Pentagon's plan to continue research on a high-yield nuclear warhead to burrow into the ground, which Mr Rumsfeld said is needed to deter countries from burying materials for weapons of mass destruction in deep bunkers.
Study already is under way on the deep earth penetrator, or "bunker-buster," but Democrats planned to try to cut the $15.5 million the Pentagon wants to continue research.