President Reagan said yesterday that the Soviet Union agreed to return to nuclear arms negotiations with the United States because "they know, as we know, that the choice now is to have some legitimate agreement on reduction of arms or face an arms race."

Reagan said the Soviets have not responded to the U.S. proposal that arms talks resume in Geneva March 5. "I don't think anyone looking back over history should be euphoric" about prospects for those talks, Reagan said, but "I just have to cling to some optimism."

He said "this is literally the first time" that the Soviets "have ever publicly stated a desire to reduce the number of weapons." For years, Soviet statements have included calls for reduced nuclear arsenals.

"Always before, it seemed we sat down and the negotiations were, 'Well, how fast are we going to increase them?' And now, here we're coming at this with both sides having said that their ultimate goal would be . . . to eliminate nuclear weapons entirely," Reagan said.

He made the comments in an interview yesterday with Helen Thomas of United Press International. In a separate interview with Michael Putzel, Terence Hunt and Charles J. Lewis of the Associated Press, Reagan said the deaths of two Soviet leaders during his presidency has "obviously" had "an effect" on Moscow's ability to negotiate arms control agreements.

"In the 48 years between [Franklin D.] Roosevelt coming into office and my administration, there were eight presidents of the United States, and in all those 48 years, there were only three leaders of the Soviet Union. Well, I had three before the first three years were up," he said.

When Roosevelt took office in 1933, the Soviet leader was Joseph Stalin. He was succeeded by Georgi M. Malenkov, Nikita S. Khrushchev and Leonid I. Brezhnev, who was in power when Reagan was inaugurated four years ago.

Asked if he intends to continue compliance with past arms control agreements, Reagan said, "Well, we have been more or less doing that.

"I think we've been doing more of it than they have in SALT II. But I just think as long as they know that, in the absence of an agreement, we are not going to sit back unilaterally disarming and let them carry their great military buildup to an unquestioned superiority. Then there would be no point in negotiating because thay'd have no reason to negotiate."

Reagan said his Strategic Defense Initiative, the antimissile defense system known as "Star Wars," would "hasten the day when we would eliminate nuclear weapons.

"What if our research revealed that we can have a defensive weapon that can, whether it is completely 100 percent effective or not, can reduce the real threat of anyone pushing the button because they know that very few of their weapons would get through? Then it just makes a lot of sense to say, 'Let's eliminate that weapon,' " he said.

"Now, if both sides have it, this answers the argument of those who say, 'Well, won't the other side just multiply the number of weapons, hoping to increase the number that could get through a defense?'

"As a matter of fact, this is why we said all we want to do right now, all we're asking, is research. And when the time comes that research leads to the development of a weapon, we're willing to meet and discuss deployment," he said.

Reagan also said he is not ready for a summit meeting with Soviet leader Konstantin U. Chernenko because "there must be some things that we're going to meet, that require a summit to discuss and talk about."

On other topics in the two interviews, Reagan said:

* "I would hope" that farmers would not go bankrupt because of cuts in price supports that he will propose in the next budget. "We're not instantly pulling the rug out from under them," he said.

In earlier years, he said, farmers not receiving government help sold more of their product on a "per capita" basis than did those who received federal aid. Reagan added, "That's why I believe" that most farmers "want a procedure that gets us back to the marketplace."

* "In the war on poverty, poverty won." He said poverty had decreased until the Great Society programs of the late 1960s and 1970s and started increasing after those programs began.

Many private analysts have disputed this view, saying poverty declined through the late 1960s, was relatively steady in the 1970s and started increasing in the 1980s. Reagan said the poverty rate is "still increasing, but we've reduced the rate of increase in that."

* He is considering "ways of capping some of the charges that can be made on Medicare patients" by doctors and hospitals to improve Medicare's financial situation.

"Before government was involved in medical care," he said, most doctors cared for some patients without charge. But "when government does step in and intervene then, in a sense, you've said" to the doctors "now they don't have to bear the burden by themselves, that all their fellow citizens are going to bear it," he said.

* He thinks that abortion is the taking of life but that, as for penalties, "we'll see what the legalities are from there," presumably if it is banned by constitutional amendment. Asked if the penalty should be capital punishment, Reagan said, "Well, I haven't thought about it from that standpoint."

* He has not decided on a proposal to create a Department of International Trade and Industry by breaking up the Commerce Department.

* He felt "cheated" by the 1982 tax bill because he thinks that Congress did not deliver all of the spending cuts it promised. "I definitely believe that increasing taxes endangers the recovery that we're havOng," he said.

* Because he is "supposed to be titular head of the party," he "won't answer" whether he is urging Vice President Bush to run for president in 1988.