Apparently confident that they will win the key test on the "loyalty rule" Monday night, backers of President Carter today signaled concessions to Sen. Edward M. Kennedy on other rules and platform issues in a bid for party unity.

As delegates began arriving in the sizzling summer weather, Kennedy hinted that he may release his supporters formally in an effort to push the "open" convention prospects.

With a full day and evening of caucuses in prospect for Sunday, there was little sign of movement on the main fight: the Kennedy-led effort to break the proposed convention rule locking delegates to their candidate pledges.

Kennedy aides said they were within 100 votes of victory on the rules issue, but the Carter side said its margin was twice that. An NBC-Associated Press poll of 2,899 delegates gave the loyalty rule a 56-to-40 percent majority.

In a move obviously designed to suggest a confidence of victory the numbers did not support, Kennedy released a list of seven possible vice presidential nominees, not all of whom, spokesman Richard Drayne said, had been contacted by the senator.

The seven names were Sen. Henry M. Jackson of Washington, former Florida governor Reubin Askew, Los Angeles Mayor Thomas Bradley, Rep. Lindy Boggs of Louisiana, Secretary of Education Shirley Hufstedler, Rep. Richardson Preyer of North Carolina and retiring Sen. Adlai E. Stevenson of Illinois. Kennedy said he also wanted to include Sen. Robert C. Byrd of West Virginia, but Byrd asked him not to.

Even as they prepared to reinforce discipline among their arriving delegates on the loyalty rule, Carter strategists were looking for concessions to give the Kennedy camp in hopes that the senator and his supporters will rally behind Carter's campaign.

Several White House and Carter campaign officials said today that the president has agreed to accept a Kennedy rules proposal requiring potential nominees to give delegates a written statement of their views on the platform including a pledge to carry out all its recommendations they do not specifically repudiate.

Officials said Carter had been resistant to that requirement, fearing it would tie his hands in the campaign and during a possible second term. But they said he was persuaded that it might be to his advantage to indicate specific areas of disagreement with the defense sections, for example.

And, tactically, they said, Carter was persuaded that it would be unwise to risk a test vote on this question Monday afternoon before the major roll call on the loyalty rule.

In addition to that concession, there were many indications that the White House was preparing to accept some of the minority planks on platform issues being pushed by the Kennedy forces.

Stuart Eizenstat, Carter's domestic olicy assistant and chief liaison with the platform committee, was in his White House office today, working out a decision on which of the 13 remaining Kennedy minority reports the Carter forces might accept.

Eizenstat is to announce those additional concessions Sunday, but informed sources said they probably would not include any of the four major economic issues scheduled for prime-time debate Tuesday.

Seeking to soften the impact of predictions Friday by Carter campaign chairman Robert S. Strauss that the president might lose some of the platform fights, Eizenstat and others vowed that the Carter forces would not deliberately "throw" any contested planks to Kennedy.

"Anything we fight, we're going to try to win," Eizenstat said. "That doesn't mean we'll win them all, but we hope to keep our losses to a minimum."

On the rules fight, the Kennedy camp is counting on a sort of "cauldron effect" to start working as delegates pour into New York from around the country. Kennedy's top political adviser, Paul Kirk, said today that this city will turn into a "hot cauldron" of rumors and news leaks and person-to-person lobbying that could change many minds quickly.

"When these people get to the convention," Kirk said, "and hear other Democrats talking about the danger to our party . . . there will be movement.And once this thing breaks, it will break heavy."

To help heat the cauldron, Kennedy may issue a formal statement releasing his 1,243 delegates from their pledge to support him.

Kennedy told a news conference today that his delegates are already free. "My delegates today are not bound," he said. "We have told them they should vote in accordance with their conscience on . . . every question." But a formal statement releasing them might draw new attention to the open convention drive, Kennedy aides say, and perhaps put a little more pressure on Carter to do the same.

The Kennedy people also say releasing their delegates is a risk-free step, because their telephone polls indicate that the Kennedy delegates will stick with their candidate whether bound or not.

The Kennedy campaign was also thinking about other moves to keep tension high on the rules question. Possibilities included private efforts to convince some anti-Carter Democrats to promote favorite-son movements in certain big states.

The theory was that the possibility of a favorite-son candidacy might induce delegates to vote for the open rule Monday so they could vote for their favorite sons in Wednesday's presidential balloting.

The favorite-son speculation increased somewhat after Kennedy met with New York Gov. Hugh L. Carey. Carey, who supports the open rule and is neutral in the Carter-Kennedy contest, would be one logical favorite-son candidate.

Kennedy and Edward Bennett Williams, the Washington lawyer heading an open convention campaign mounted by a group of Democratic members of Congress, have both asked Carey to speak in favor of the open rule during the rules debate Monday. The governor's staff said today he would probably do so.

Kennedy worked late Friday night and again today on drafts of the speech he is to deliver to the convention during the platform debate Tuesday.

Although that speech cannot be finished until the result of Monday's rules fight is known, the heart of it, no matter what happens, will be a forceful restatement of Kennedys liberal position on economic issues according to campaign staffers.

The ghostwriters helping Kennedy on this important speech are Robert Shrum and Carey Parker, two steadfast liberals who have shown no personal inclination toward embracing Carter in the name of the party unity.

In other speeches written for him recently by Shrum and Parker, Kennedy has made a conditional offer of unity: he can support the Democratic nominee if the nominee supports the liberal economic measures Kennedy has espoused.

Kennedy spent most of today in a Waldorf-Astoria suite working on the speech and calling delegates to ask for help on Monday's vote. He took time off in the morning, though, for a classic media event: an interview with 19 children aged from 8 to the low teens who are putting out a newspaper here called "Childrens Express."

The children, wearing bright yellow T-shirts, sat Indian-style on the floor and quietly read their questions to Kennedy, who responded with long answers identical to those he gives his grown-up press corps. Meanwhile, a dozen television crews and three dozen still photographers hovered around to capture the moment on film and tape.