Key members of Congress said yesterday that President Reagan will be denied the new military aid he is seeking for El Salvador unless he agrees to some softening of policy there, and the administration last night was studying possible new steps to appease its critics.

Rep. Clarence D. Long (D-Md.), chairman of the controlling House Appropriations subcommittee on foreign operations, has told Secretary of State George P. Shultz that he will not endorse the administration's request to shift $60 million in already appropriated funds to El Salvador unless the administration meets five conditions.

The most important of these is that the administration press for inclusion in the forthcoming Salvadoran election of members of the Christian Democratic and Social Democratic parties who are in exile and have been supporting the guerrillas there. Long said he wants something more than "verbal assurances" that the exiles will be able to return to El Salvador and campaign and vote safely.

Steps must also be taken, Long said, to strengthen the Salvadoran judicial system to assure punishment of those guilty of human rights abuses and murders of U.S. citizens and other civilians in that country.

Long's 14-member subcommittee has veto power over Reagan's so-called reprogramming request, the leading edge in $298 million in military and economic aid for Central America that the president called for in a speech on Thursday. The panel must act within 15 days.

The Senate Appropriations subcommittee on foreign operations and the Senate Foreign Relations Committee could also veto the request, but are thought to be more favorably disposed than the Democratic-controlled House subcommittee.

Asked about Long's conditions, a State Department spokesman said, "We are aware of Chairman Long's and the committee's concerns and are addressing them," and the administration yesterday was preparing a document it hoped would meet Long's terms.

"There's no question Mr. Long is a very powerful man in this situation," said a senior administration official who asked not to be identified. "He holds his finger on our jugular vein in this."

Long said yesterday that, after a trip to El Salvador last month, "I shifted from being totally opposed to more military aid to being open-minded. But the administration has to understand that without a move toward a broad-based political solution, they're not going to get the money. I'm not going to support throwing money down a bigger and bigger rathole."

Long said Shultz had assured him that the administration "will have something definite, not rhetoric" by next week and would give Long a draft of new policy initiatives before making them public. "Shultz said he would come through with some really hard stuff," Long added.

Administration officials said that Reagan favors the same goals as Long, and said so specifically in his Thursday speech. But, the officials added, there are limits on the degree to which Washington can dictate terms to the Salvadoran government, and they were unable to predict whether the document being prepared will satisfy Long.

The exiled Christian Democrats and Social Democrats who have been functioning as the political arm of the Salvadoran revolutionary movement must "have real assurances that they won't get killed" if they return to take part in the elections, Long said.

He added that the Salvadoran military must show a willingness to follow the advice of its American trainers and move from defensive holding actions to a more aggressive strategy of using small, mobile units to actively search out the guerrillas.

Shultz breakfasted with two key members of Long's subcommittee yesterday, Reps. Matthew F. McHugh (D-N.Y.) and Julian C. Dixon (D-Calif.), who hold swing votes on the generally liberal panel. McHugh said in an interview afterward that he will vote against the aid request unless "the president makes a stronger public statement that our government is going to pursue meaningful negotiations."

The president's speech Thursday "did not put enough emphasis on an attempt to reach a political settlement," he said, adding that negotiations with the guerrillas should focus on a cease-fire during the elections and the disposition of forces, plus security guarantees to make the elections credible.

McHugh said Shultz was "a good listener," but did not commit the administration to going beyond the president's speech, which endorsed negotiations in general terms without specifying what steps might be taken to bring them about.

The extent to which Reagan's Salvadoran policy is in trouble in Congress also was reflected by the early stance of another key Democrat, Appropriations Committee Chairman Jamie L. Whitten (D-Miss.), who provided a swing vote in favor of Salvadoran aid last year.

In an interview that reflected frustration felt by many members, Democrats and Republicans, Whitten said that "I would have to be convinced beyond where I am now" to vote for the aid this year. "Before, I thought the president deserved a chance. But we've had two years, and I can't see that things have improved. We all remember Vietnam. I don't know that this money is contributing anything other than getting us deeper and deeper involved."

Even the Republican-controlled Senate is restive. Sen. Robert W. Kasten Jr. (R-Wis.), chairman of the Senate Appropriations subcommittee on foreign operations, said he had scheduled a hearing tentatively for March 23.

Kasten said the subcommittee probably would consider attaching conditions to approval, including a prohibition against sending more U.S. advisers to El Salvador, and a requirement that the Salvadoran government enter into some kind of negotiations with the guerrillas.

Appropriations Committee Chairman Mark O. Hatfield (R-Ore.) introduced legislation this month to cut off military aid and require the withdrawal of U.S. advisers unless the government of El Salvador makes efforts in good faith to start talks with the rebels. Hatfield, a member of the Kasten subcommittee, is likely to push to attach his bill to the reprogramming request.

Meanwhile, Sens. Christopher J. Dodd (D-Conn.) and Nancy Landon Kassebaum (R-Kan.) released a letter to Senate Foreign Relations Committee Chairman Charles H. Percy (R-Ill.) requesting a hearing and a vote on the reprogramming request there.

"I think there won't be a total acceptance of the president's request," Kassebaum said. "But we have to find a level that still shows some support" for the Salvadoran government.