The Senate yesterday passed by voice vote a $20.7 billion energy and water appropriations bill for 1991, focusing its debate chiefly on a controversial amendment to reimburse an Idaho rural electric cooperative $1 million to cover a lost tax credit.

The amendment, sponsored by Sen. Jim McClure (R-Idaho), passed on a roll call vote of 76 to 23. Supporters lobbied for Democratic support on grounds that the measure would help the reelection bid of Sen. Max Baucus (D-Mont.). The $1 million was considered essential to keeping on track a hydroelectric project that will supply power to parts of southwest Montana.

But the political argument failed to win over Appropriations Committee Chairman Robert C. Byrd (D-W.Va.) and Sen. J. Bennett Johnston (D-La.), chairman of the energy and water appropriations subcommittee that drafted the main legislation. Both voted against the amendment.

The project, to be carried out by the Bonneville Pacific Corp., adds a hydroelectric power turbine to an existing dam on Henry's Fork of the Snake River in Idaho. As a result of congressionally mandated environmental requirements, the Federal Energy Regulatory Commission delayed issuing a final license until the 1986 tax reform act removed a tax credit for which the project would have been eligibile earlier.

The contractor and the Fall River Rural Electrical Cooperative have since said the project will not go ahead unless they receive reimbursement. The reimbursement, opposed by the Bush administration and unauthorized by the Senate Energy Committee, is listed in the amendment as "reimbursement for environmental protection requirements."

Supporters said Senate action amounted to "equitable relief" for the Fall River Rural Electrical Cooperative. But one congressional critic described it as a "terrible amendment" that singles out one taxpayer for relief from the impact of tax reform.

The overall measure finances nuclear weapons production, nuclear waste cleanup, river and harbor projects and some $4 billion worth of research on nuclear and other sources of energy. Johnston used the Iraq crisis to warn about the country's increasing dependence on energy imports.

Wednesday night, Sen. Rudy Boschwitz (R-Minn.) dropped a plan to seek a 2.8 percent cut in the bill after being told "I had a very good chance of having my projects {in Minnesota} targeted" if he proceeded.

"It's a pretty rough business," he said.