President Clinton may face an impossible choice after a planned Senate vote Wednesday on a measure to punish Russia for its nuclear sale to Iran.

The administration is watching the Senate's $12.3 billion foreign operations appropriations bill closely. Not only does the bill slash $2.4 billion from Mr. Clinton's overall foreign aid request, it contains a controversial provision that would bar funds for Russia unless Mr. Clinton can certify that Moscow has dropped plans to sell nuclear reactors to Iran.The measure sponsored by Sen. Mitch McConnell, a Kentucky Republican and chairman of an Appropriations subcommittee, has put the president in a bind, an administration official said. If it becomes law, relations with Russia will sink to a new low. If Mr. Clinton feels forced to veto it, he can be accused of going soft on Iran.

The McConnell legislation would withhold an estimated $250 million in U.S. assistance for Russia in fiscal 1996, if it continues with its planned sale of reactors for Iran's Bushehr nuclear power plant.

Russia has resisted U.S. efforts to block the deal, although it has taken steps to guard against the use of spent nuclear fuel for weapons development. While the administration continues to oppose the sale publicly, officials say it has accepted the safeguards and inevitability of the transfer.

Like other proposed legislation on Iran and Cuba this year, the McConnell amendment seeks to manage Mr. Clinton's conduct of foreign policy. But more than other similar measures, it has relied on the administration's own statements in opposing the nuclear sale.

"They oppose it, but they haven't backed it up with action," said one McConnell staffer.

Now that Russian deliveries of reactors to Iran are in the works, there are signs that the administration has come to regret some of the strong language it used regarding the Iran threat over the past year.

"To a certain extent, rhetorically, we painted ourselves into a corner by engaging in some unnecessary hyperbole. Now, McConnell is going to paint us further into it," said an administration official, speaking on condition of anonymity.

Fortunately for Mr. Clinton, there are still several possible potholes that the measure may fall into before it reaches the level of a veto choice. The provision could be amended or withdrawn on the floor, although open opposition would risk sniping by anti-Iran hawks and pro-Israel forces. The Senate bill provides nearly $3.1 billion in military, economic and refugee aid for Israel, Congressional Quarterly reported.

If the McConnell provision passes intact with the entire bill, it could disappear in conference committee during reconciliation with the House version. The president also could veto the entire foreign aid bill without specific reference to the McConnell measure, although that would leave countless programs around the world without funds.

Then, there is the popular "train wreck" scenario, under which all appropriations and funding legislation may be endangered because of the standoff over debt limits and spending cuts.

The administration has criticized other pieces of the Senate appropriations bill, including limits on energy aid to North Korea to reduce its nuclear threat. But officials have steered clear of confronting the McConnell provision head-on and have not opposed the entire bill.