Omitting $36.6 million from a huge $388 billion spending bill it passed over the weekend, lawmakers on Monday said the technology would encourage other nations to go nuclear.
Such bunker-busting nuclear weapons, packing half the power of the bomb dropped on Hiroshima in the second world war, would be used to destroy underground facilities as well as smaller nuclear arms.
The White House said it had no plans to build such weapons, but wanted to keep the door open to their development to deal with emerging threats.
Democrats said just considering the weapons takes nuclear warfare out of the realm of the unthinkable.
“If we are to convince other countries to forego nuclear weapons, we cannot be preparing to build an entire new generation of nuclear weapons here in the US,” said Representative Ed Markey, a Massachusetts Democrat.
Markey said this “should send a strong signal to the Bush administration to drop this weapon from next year’s budget request”.