An eight-member U.S. congressional delegation warned Salvadoran political parties today that they risk losing American aid unless they form a broad coalition including the centrist Christian Democrats.

Some of the congressmen, including members of key committees, reported at the end of their 24-hour visit that the prospects of such a coalition--which appeared slim only yesterday--had now brightened.

The U.S. delegation "expressed with great candor our feeling that now is the time when they must bury the hatchet," House Majority Leader Rep. James C. Wright Jr. (D-Texas) told an informal press gathering. He said he had found "growing awareness" among the politicians that they should get together, and he was optimistic the parties would work something out.

Yesterday, Christian Democrats held out little hope that they and the four rightist parties that won a majority in the 60-seat constituent assembly elected last month could form a coalition. The Christian Democrats pictured the bargaining process until now as an effort to agree on the form in which programs instituted by the current Christian Democratic-military junta would be put before the assembly--in effect, relegating that body to rubber-stamp status.

But the Christian Democrats said they were preparing to assume an opposition role with their 24 seats in the assembly, thereby forcing into the open the bargaining on such issues as the survival of the junta's agrarian and banking reforms.

Nevertheless, Wright said today: "I get the distinct impression that the talks have taken a turn for the better."

A delighted diplomat said later, "That's not intervention. That's money" talking. Congress has been asked to provide $165 million in military and economic aid to El Salvador and will be debating much more in the coming months. The delegation included members of the House Ways and Means and Appropriations Committees.

Insisting that the group was not dictating anything to anybody but only offering guidance to El Salvador when asked for it, Wright said they had indicated to the politicians during meals and meetings that Congress "would be much more likely to give enthusiastic support" to a provisional government here "that encompassed in its makeup, in roughly proportional ways, the various parties" that won seats.

"Something happened yesterday," said Rep. E. Clay Shaw (R-Fla.), the only Republican on the trip. "Everybody was much more upbeat last night." Sources close to the talks said that one major factor was a reduction in the number of negotiators from the parties so that only six people were talking, two from each of the three largest parties. Before that, 28 people had been trying to reach agreement.

"These top guys all know each other real well. They can't bluff each other. They've been able to see that there are more areas of agreement than they had previously thought," said one observer.

Another factor was the arrival of the congressmen. "Oh, yes, they put the heat on us," said an official of one of the leading rightist groups. "But they had some fundamental misunderstandings about our willingness to talk. We straightened them out on that."

Shaw put it differently: "They were all very well aware of what we wanted to hear, and they gave it to us."

Wright said the delegation had also indicated its hope that the provisional government would produce a timetable for presidential elections "in the foreseeable future . . . not four or five years away."

The congressmen also urged that the new government construct a workable amnesty program that could bring some of the estimated 6,000 leftist guerrillas back into public life, Wright continued. Rep. Michael D. Barnes (D-Md.), chairman of the inter-American affairs subcommittee of the House Foreign Affairs Committee, said he would be "awfully cautious" if he were a guerrilla "about participating in an amnesty program without some definite changes in the society."

Rep. Dennis E. Eckhart (D-Ohio) added that the delegation had "made it very clear" to the Salvadoran politicians that "the United States is going to want to see a form, pattern and method" for controlling death-squad slayings of suspected leftists.

The Salvadoran armed forces remain "expressly uninvolved in the political negotiations, by their own choice," Wright said, relating the substance of the group's meeting with Defense Minister Jose Guillermo Garcia.

Other members of the delegation included Reps. George Miller of California, William Lehman of Florida, Clement Zablocki of Wisconsin and Charles B. Rangel of New York. Most of the group left today.