Administration officials have maintained that the country needs to try to develop a nuclear warhead that would be capable of destroying deeply buried targets including bunkers tunneled into solid rock.
But opponents said that its benefits are questionable and that such a warhead would cause extensive radiation fallout above ground, killing thousands of people. And they say it may make it easier for a future president to decide to use the nuclear option instead of a conventional weapon.
The Senate early Friday voted 53-43 to include $4 million for research into the feasibility of a bunker-buster nuclear warhead. Earlier this year, the House refused to provide the money, so a final decision will have to be worked out between the two chambers.
The money is included in a $31.2 billion spending measure for the Energy Department and other programs. Last year Congress killed the program, but the Bush administration asked that it be revived.
Supporters of the program said the $4 million does not signal development of any new warheads. They argue that the money would be used to see whether a sufficiently hardened casing could be developed for an existing warhead.
But Sen. Dianne Feinstein, D-Calif., one of Congress' most vocal opponents of the bunker-buster, said the program ''sends the wrong signals to the rest of the world by reopening the nuclear door and beginning the testing and development of a new generation of nuclear weapons.''