President Reagan said today that the United States had "unilaterally disarmed" in the face of a Soviet arms buildup before he took office, and he laid the responsibility on Walter F. Mondale and the Carter administration.

In a preview of his expected lines of attack in Sunday's foreign policy debate, Reagan also criticized Mondale for the Iranian hostage crisis, suggested that Mondale was naive about the Sandinista revolution in Nicaragua and said Mondale had "failed to repudiate" Jesse L. Jackson's sympathetic comments about Cuban dictator Fidel Castro.

Reagan opened fire on the Democratic nominee in response to student questions at Bolingbrook High School near here.

"Now, four years after our efforts began," Reagan said, "small voices in the night are scurrying about, sounding the call to go back -- go backward to the days of confusion and drift, the days of torpor, timidity and taxes."

Responding to a question about his efforts to negotiate with the Soviets, Reagan said "we're making every effort" to do so, and added:

"What we inherited when we came here was an America that over the years had unilaterally disarmed. The administration before ours, they canceled the B1 bomber. They said, 'No, we won't build it.' They didn't ask anything from the Soviet Union in the terms of arms control or anything."

Reagan noted that the SALT II treaty had been signed but said, "All that was, was legitimizing a continuation of the arms race."

"Since that treaty was signed, they've added 3,800 more warheads to their arsenal, nuclear warheads," Reagan said. "And, incidentally, my opponent was the president of the United States Senate at the time, and a Democratic Senate under a Democratic administration refused to ratify that treaty for the reason I just gave."

Reagan repeated his contention that the Soviets would negotiate arms reductions "if they see that the United States is willing to go as far as it has to go to see that they don't stay ahead of us in weapons -- that we're as strong as they are."

Later, speaking at the College of DuPage in this Chicago suburb, Reagan said:

"After the hostages were taken in Iran, my opponent said it would be 'a temporary problem.' Later, he called his administration's handling of the affair 'masterful.' "

Reagan did not elaborate, but aides said the remark that Iran was a temporary problem came Dec. 12, 1979, a month after the American diplomats were seized. The "masterful" remark was made Jan. 18, 1980, Reagan aides said. The hostages were released a year later.

On Nicaragua, the president said: "After the Sandinista revolution in Nicaragua, my opponent said the 'winds of democratic progress are stirring where they have long been stifled.' That was right before the Sandinistas slaughtered the Miskito indians, abused and deported church leaders, slandered the Holy Father and moved to kill freedom of speech."

Reagan aides said the Mondale remark was reported in a State Department bulletin Nov. 1, 1979, 3 1/2 months after Nicaraguan dictator Anastasio Somoza fled his country and the first Sandinista cabinet was set up in July 1979.

Although the Sandinistas later turned away from democracy, Mondale's statement reflected a view widely held at the time in the United States that their revolution might lead to elections and be less repressive than the Somoza regime.

"More recently," Reagan said today, "my opponent failed to repudiate the Rev. Jesse Jackson when he went to Havana, stood with Fidel Castro and cried, 'Long live Cuba.. . . Long live Castro. . . . Long live Che Guevara. . . .'

"And my opponent has never disassociated himself from that kind of talk," Reagan concluded.

Jackson's full quote was: "Long live Cuba. Long live the United States. Long live Castro. Long live Martin Luther King Jr. Long live Che Guevara. Long live Patrice Lumumba. And long live our drive for freedom."

Asked today whether his age would prevent him from serving another term, Reagan declared, "I've never felt better in my life."

He joked: "The way I put it is, I'm not really this old. They mixed up the babies in the hospital."

Reagan told the high school audience today he got his childhood nickname "Dutch" from his Irish father, who was commenting on a haircut his mother had given him.

The story conflicts with the one Reagan told in the first paragraph of his 1965 autobiography, "Where's the Rest of Me?" in which he says his father gave him the nickname at birth because he looked like "a fat Dutchman."