Last-minute obstacles are holding up legislation on the disposal of low-level radioactive waste, raising the possibility that many medical facilities and nuclear power plants will have no place to ship their radioactive garbage after Jan. 1.

At issue is a measure that would give states until 1993 to develop disposal sites for low-level radioactive waste, including everything from laboratory leftovers to radioactive wastewater and contaminated clothing from nuclear plants.

A 1980 law gave the states until the end of this year to find ways to handle their own low-level nuclear waste, either individually or jointly with neighboring states. None will meet the deadline.

As a result, South Carolina, Washington and Nevada -- the only states with operating facilities for low-level radioactive waste -- say they will close their sites to outsiders unless Congress approves an extension of the deadline with enough teeth to force the other states to act.

Loss of access to the existing sites would leave dozens of nuclear power plants and thousands of hospitals, clinics and laboratories with about 2.8 million cubic feet of radioactive waste on their hands each year.

"If that bill crashes, we have a crisis on our hands," said Wright Andrews, a Washington lawyer representing the three states with disposal sites.

Earlier this year, the House overwhelmingly approved a bill that would set a series of step-by-step deadlines for states to follow in developing sites and impose stiff penalties on any that failed to comply. The bill also would limit the volume of waste South Carolina, Washington and Nevada would be required to take, and more than double the cost of sending waste to those sites in the next several years.

But the Senate has yet to consider the bill, largely because Sen. Daniel J. Evans (R-Wash.) is holding out for stronger penalties on states that fail to meet its deadlines.

An aide to Evans said yesterday the House legislation and a nearly identical version crafted by the Senate Environment and Public Works Committee contain "loopholes" that would let recalcitrant states keep dumping their waste in Washington longer than Evans believes is necessary. Evans favors a tougher approach approved by the Energy and Natural Resources Committee, of which he is a member.

The environment committee version would grant leeway to states unwilling to handle radioactive waste in shallow landfills, the only practice for which the Nuclear Regulatory Commission has developed rules. Arguing that such landfills may not be safe in states with high water tables and abundant rainfall, the panel wants to grant those states an extra year to use existing facilities while waiting for the NRC to develop regulations on other radioactive waste disposal methods.

Evans contends the NRC will have the rules ready well in advance of the cutoff deadline, and the "grace period" only will encourage states to put off the politically explosive issue of developing a nuclear-waste dump.

Negotiations over the weekend failed to resolve the impasse, dimming chances that Congress will enact the measure this year. The stalemate jeopardizes more than a year of delicate negotiations to solve the problem of low-level nuclear waste disposal, backers say.

"If Congress has to come back to it, these deals are probably blown apart and we have to start over from square one," said Andrews, noting the states with operating sites remain adamant about barring out-of-state waste. "It's going to be a bloody fight," he said.