Senate leaders predicted yesterday that Congress will cut back President Reagan's request for $110 million in military aid to El Salvador and probably will attach conditions that a senior official decried as "designed to give us the worst of both worlds" in pursuing an end to the Salvadoran conflict.

With a showdown looming next week on the first part of Reagan's request, Senate Majority Whip Ted Stevens (R-Alaska) said he believes "some additional assistance will be approved." But he added that the Senate is likely to trim the amount from what the administration has requested.

"I don't think they will. I think we will have to," Stevens replied to questions about whether the administration will lower its request in hopes of overcoming strong congressional opposition.

A similar view came from Sen. Charles H. Percy (R-Ill.), chairman of the Senate Foreign Relations Committee, who said that "Congress isn't going to go along with that large an amount" and probably will give the administration "something over half" of what it wants.

While stressing that he supports the full $110 million request, Percy also said he doesn't think the Salvadoran civil war "is winnable militarily . . . . Our best effort is to bring about a negotiated settlement."

He predicted that Congress will insist on attaching conditions to the aid to put pressure on the Salvadoran government to seek a negotiated political settlement with its guerrilla foes.

In that respect, considerable attention was being focused on a proposal put forward by Sen. Nancy Landon Kassebaum (R-Kan.) that is expected to gain the support of two Democratic senators critical of administration policy: Daniel K. Inouye of Hawaii and Christopher J. Dodd of Connecticut.

The three began conferring yesterday, and Kassebaum said they expect to make a joint statement on Monday that she hopes will serve as "a rallying point" for those who want to force the administration to alter its course in Central America.

Kassebaum's proposal, which she outlined late Thursday, would trim Reagan's request to $23.7 million and hold military aid for El Salvador to $50 million in the next fiscal year. It also would limit the number of U.S. military advisers in El Salvador to 55, and it would put the Salvadoran government on notice that future aid would depend "in large part" on agreeing to talks with the guerrillas "without any preconditions."

Sen. Mark O. Hatfield (R-Ore.), chairman of the Senate Appropriations Committee, called Thursday for Reagan's request to be cut substantially and for all aid to be halted unless the Salvadoran government agrees to unconditional talks.

These signs of mounting resistance came as the administration fought to convince Congress not to interfere with the first part of its request, which involves diverting $60 million in funds appropriated for other countries to El Salvador.

However, the reprogramming will be blocked if the Senate and House Appropriations subcommittees on foreign operations or the Senate Foreign Relations Committee vote against it next week.

A senior State Department official who has been involved in the congressional lobbying effort made clear yesterday that the administration regards the calls from Kassebaum and Hatfield for unconditional negotiations as unacceptable because such talks might open the way for the guerrillas to demand a power-sharing plan, without elections as advocated by the United States.

The official, who spoke on condition he not be identified, said the guerrillas have called for a "dialogue without preconditions . . . which is the biggest single condition you can have" because it would allow the guerrillas to demand key government posts for communists.

He expressed hope that "the American people are not going to be bound by the Vietnam syndrome for the rest of history" and asserted that, while the administration wants a bipartisan approach to El Salvador, "some proposals in Congress seem designed to give us the worst of both worlds" in terms of pursuing the fight against the guerrillas and building democracy there.