President Reagan, in high spirits after his debate with Democratic nominee Walter F. Mondale, today opened the final two weeks of his campaign by sharply attacking Mondale's defense record.

Speaking at the Rockwell International assembly plant where the B1 bomber is built, Reagan said, "If it were up to my opponent, I'm afraid Rockwell might still be building the B25 -- that is, if you were building anything at all."

The B25 is a World War II-vintage bomber.

"The truth is, if all his votes had prevailed in the Senate , America would barely have any defense, any real means to protect the peace, any chance to preserve freedom -- and we can't afford that kind of protection," he said.

Reagan still was dogged by questions today about his 1982 news conference statement suggesting that he believed that submarine-launched nuclear missiles could be recalled. Mondale criticized Reagan in the debate for the remark, and Reagan tried again today to explain it.

But these questions did not dampen the buoyant mood in the Reagan entourage after the debate. Reagan political adviser Lyn Nofziger said Mondale had lost his "last shot" at defeating Reagan. White House chief of staff James A. Baker III said Mondale "didn't do anything to make up the gap in polls he's going to need to make up."

Reflecting the campaign's upbeat mood, Nancy Reagan cheerily told reporters in a rare conversation on Air Force One that questions about the president's age and competence are "a non-issue. It's not true and he's shown it's not true."

Picking up where he left off in the debate, Reagan said Mondale "voted to delay or kill virtually every new strategic system: the B1 bomber, the Trident submarine and the Trident missile, the cruise missile, our ABM system, and the modernization of our ICBM force."

Mondale's record on conventional forces is "just as frightening," Reagan told several thousand aerospace workers, saying that the Democrat has expressed "hostility to a strong, secure America" and that his election would "send a signal of decline, lessened will and weakness to friends and adversaries alike."

"It's always easy to argue for reductions in defense spending, just as it's easy to pretend that one can call the Soviet leaders, as my opponent has proposed, and persuade them in a moment to alter the course they have followed for decades," Reagan said.

"It's also nonsense and the American people know it," he added.

Reagan restored the B1 bomber program canceled by President Carter. The Pentagon is buying 100 bombers, designed for treetop-level penetration of enemy air defenses, at an estimated cost of $28.3 billion. Critics say advances in defensive measures will make the plane obsolete before the 100 are delivered in April 1988. Mondale says he supports development of the Stealth bomber instead of the B1B.

The president spoke today against the backdrop of a B1B under assembly and a huge banner, "Prepared for Peace." He made no mention of the fatal crash last month of a B1 prototype, which has been linked to pilot error, but did note last week's successful initial flight of the first production model.

On the question of whether submarine-launched missiles can be recalled, Reagan said today, "I was talking about the submarines and the airplanes, that they could be called back."

Referring to Mondale's criticism of the remark, Reagan said, "How could anyone think that any reasonable human being could believe that you can turn a missile around and bring it home? I think that shows a lack of intelligence on their part."

The president complained that his remark "was taken out of context and I realize that all of you reporters are on the record of having repeated it so often, but now try and find a way back off the end of the limb. I won't cut it off."

At a May 1982 news conference, Reagan said there was "no recall" and "no defense" for land-based missiles.

But he added, "Those that are carried in bombers, those that are carried in ships of one kind or another, or submersibles, you are dealing there with a conventional type of weapon or instrument, and those instruments can be intercepted. They can be recalled if there has been a miscalculation."

Asked what he thought of Mondale's performance in the debate, Reagan said, "I thought he was his usual self.