A key Democrat yesterday predicted that President Reagan's request for an immediate $60 million in additional military aid to El Salvador will be cut to $23.7 million in the Senate Appropriations subcommittee on foreign operations.

Sen. Daniel K. Inouye (Hawaii), ranking Democrat on the subcommittee, yesterday endorsed a proposal by Sen. Nancy Landon Kassebaum (R-Kan.) to cap military aid to El Salvador at $50 million a year until progress is made toward opening unconditional negotiations with the guerrillas. That would add $23.7 million in new aid on top of the $26.3 million already appropriated for this fiscal year.

"We're not abandoning" the Salvadoran government, Inouye said at a news conference.

"The administration has every opportunity to submit a supplemental request, and then, if the administration can convince us that negotiations are going on and the Salvadoran government has achieved some stability, this Congress could become a bit more generous," he said.

Reagan has proposed $110 million in extra military aid to El Salvador this year, of which the first $60 million would be shifted from other countries.

This so-called reprogramming request can be vetoed by the Senate or House Appropriations subcommittees on foreign operations or by the Senate Foreign Relations Committee. The remaining $50 million would be a supplemental appropriation that would have to go before the full House and Senate.

Inouye said he will make a motion today to cap the aid after Secretary of State George P. Shultz' scheduled testimony before the subcommittee this morning.

"If I know my math, it seems to me we have the votes," Inouye said.

The House subcommittee, headed by Rep. Clarence D. Long (D-Md.), plans to meet in closed session on the request today and to vote Wednesday. Sentiment in the Long unit has been running strongly against increasing military aid unless progress toward a political solution is in sight.

The Kassebaum proposal, also endorsed by Sen. Christopher J. Dodd (D-Conn.), who appeared at the news conference with Inouye and Kassebaum, would:

* Cap military aid in fiscal 1983 and 1984 at $50 million a year and limit economic support to $105 million.

* Prohibit supplementation of the aid without congressional approval.

* Cap the number of U.S. advisers in El Salvador at 55 through fiscal 1984.

* Encourage an "open dialogue without preconditions" among opposing parties in El Salvador.

* Call for an immediate conference of all Central American governments to encourage a regional approach to resolving the Salvadoran conflict.

The idea of unconditional negotiations has been sharply opposed by administration officials, who want to limit discussions between the Salvadoran government and the rebels to the subject of the December elections. Assistant Secretary of State Thomas O. Enders testified last week that the administration would make detailed proposals toward guaranteeing the security of and access to the media for members of the opposition who choose to take part in the elections.

So far, however, the guerrillas have been given no concrete guarantees of security during an election and have ruled out participating in the elections. The offer of unconditional negotiations, Dodd said, will "test the credibility of the insurgents." Kassebaum added that if the administration makes "a good-faith offer" and the guerrillas refuse to negotiate, Congress would be likely to provide "further assistance" to the Salvadoran government.