President Reagan last night appealed to the Soviet Union to join with the United States in negotiations "to substantially reduce nuclear weapons and make an important breakthrough for lasting peace on earth."

In the first prime time news conference of his administration, the president coupled his appeal for nuclear arms bargaining with a rejection of proposals for a nuclear arms freeze, which he said would be "disadvantageous, in fact even dangerous" to the United States.

The president contended that the Soviets have "a definite margin of superiority" in nuclear weaponry and asserted that any freeze would maintain this purported Russian advantage. Reagan said that the Soviets would be able to absorb a U.S. retaliatory attack "and hit us again."

At the same time Reagan declared that no one could win a nuclear exchange.

"I just have to say that I don't think there could be any winners; everybody would be a loser if there's a nuclear war," he said.

The president's news conference was intended by his advisers to give him an opportunity to talk directly to the American people at a time when his poll ratings are slipping and he is under attack in Congress on his domestic and foreign policies. Last night the president repeatedly attempted to defuse criticisms that his policies are unfair to the poor. On other major issues, the president:

Said that the United States would be concerned if a right-wing government is formed in El Salvador that abandons social reforms. "I think that it would give us great difficulties if a government now appeared on the scene that totally turned away from the reforms that have been instituted," Reagan said.

Embraced, as he has in the past, the "concept" of a constitutional amendment requiring a balanced budget. At the same time he refused to consider a rollback of his present tax reduction program, which has added heavily to federal deficits, saying that taxes should not be increased during a recession.

Expressed hope that progress could continue toward Palestinian autonomy talks despite recent clashes between Israeli military forces and Palestinians on the West Bank. Reagan said he was "hopeful that we will see more progress on these talks" after Israel completes transfer of the Sinai to Egypt on April 25.

Said that the U.S. government is closely watching the situation in Poland and "that we are doing everything we can to try to help the Polish people" without having it appear that the Polish government is providing $55 million in grain and corn that was sent by the United States. "I think that it is also necessary that they understand that there could be a carrot along with the stick if they straighten up and fly right," Reagan said.

Reagan's principal objective in his comments on foreign policy issues last night was to defuse mounting criticisms that he is uninterested in meaningful negotiations with the Russians to reduce nuclear weapons.

The president said in response to the last question of his news conference that he was hopeful that the negotiations could begin possibly as early as this summer. They are expected to be an important topic for the president and European leaders when Reagan visits Europe in June.

Reagan's statement that the Soviets have nuclear superiority goes beyond the usual statements that have been made by officials of his administration on this subject. The officials have said that the United States no longer has superiority but they rarely, if ever, have contended that the Soviets have an overall advantage.

During his 1980 presidential campaign, however, Reagan frequently made the assertion that the Soviets had obtained a strategic advantage over the United States.

In his initial news conference as president in 1981 Reagan had referred to the Soviets as "liars" and "cheats." Last night in his opening statement he called for "a return to the level of civilized behavior we once knew" but said later that he hadn't changed his opinion of the Russians.

"I don't think they've changed their habits," Reagan said. "I think, however, they're in a more desperate situation than . . . I had assumed that they were economically. Their great military buildup has, at the expense of denial of consumer products up to and including food for their people, left them on a very narrow edge . . . . "

In an opening four-minute statement Reagan basically embraced a bipartisan proposal by Sens. Henry M. Jackson (D-Wash.) and John W. Warner (R-Va.) "calling for major verifiable reductions of U.S. and Soviet nuclear weapons to equal force levels.

"This is an important move in the right direction and these points are essential elements of a truly effective arms control agreement--elements which are consistent with the views of this administration," Reagan said.

"America's national security policy is based on enduring principles," he said. "Our leaders and our allies have long understood that the objective of our defense efforts has always been to deter conflict and reduce the risk of war, conventional or nuclear."

Reagan was asked what he thought of Soviet President Leonid Brezhnev's proposal that both sides freeze the placement of intermediate-range missiles in Europe at the present levels, which the administration has criticized on the grounds that the Russians have almost all the new missiles in place:

"We'll have to see what the statements mean," the president said. "This is just part of the dialogue, the propaganda campaign to make them look like the peacemakers and that we're the ones seeking war."

Reagan's advisers, who have been downcast after some recent press conferences because of presidential stumbles and misstatements, were pleased that he had emerged from last night's prime time performance apparently unscathed.

In an unusual move, his prepared statement was not distributed in advance to reporters gathered in the East Room because as one aide put it, "we didn't want the press to get an advance look at it and get a chance to pick it apart."

Reagan turned one question at the press conference to his political advantage when he was asked whether the trip he would make to Barbados next week would enhance an "image of you as a rich man's president."

He observed that a bipartisan congressional team is visiting Caribbean islands at the same time and said that he would be meeting with heads of state during the five days he is out of the country.

"Now it is Easter and everybody else is taking a vacation," Reagan said. "The fact that while we are there I am going to sit in the sand and maybe go swimming for a day before we come back hardly constitutes what I would consider a vacation."