Even as it strives to keep nuclear weapons from proliferating around the world, the Bush administration is moving toward research on a new generation of less powerful nuclear warheads. That effort, recently endorsed by Congress, unwisely overturns a decade of restraint intended to discourage development of a new nuclear arms race.

The new weapons are portrayed as a way to meet emerging threats that the existing nuclear arsenal, aimed at obliterating the Soviet Union in an all-out war, was not designed for. Some would be relatively small, low-yield weapons that could be used against a variety of targets, ranging from mobile targets to underground bunkers. Others would be even larger bunker-buster warheads.

The trouble is that the smaller weapons might be tempting to use in situations where no one would dream of dropping a more massively destructive nuclear bomb. That could speed the end of the “nuclear taboo” that has kept the world free of nuclear warfare since World War II.

For the past decade, design and development of the smaller weapons, with a yield below five kilotons, has been banned in this country by law. The goal was to keep from blurring the distinction between nuclear and conventional weapons by lessening the difference in their destructive power. This year the Bush administration asked that the ban be lifted, and both the Senate and House passed bills authorizing research to proceed while requiring further Congressional approval before moving to development or production.

Nuclear proponents argue that rogue nations are burying command centers and facilities to make nuclear, biological and chemical weapons underground, often in hardened structures that are difficult to destroy. But even a small nuclear weapon detonated below ground would spew out a mass of radioactive material. Moreover, any president would need to have extraordinary confidence in intelligence assessments about underground facilities before ordering a nuclear strike. Given the difficulty in finding weapons of mass destruction in Iraq, such confidence may be hard to come by.

Instead of creating a new generation of nuclear warheads, Washington should concentrate on improving its precision-guided bombs and missiles that carry conventional warheads. Administration officials insist that they are only doing research and are not committed to developing new weapons, but this project could well become the opening wedge for a full-fledged production program. Congressional opponents of a nuclear arms race should make sure that this effort stops at the research stage.