Vice President Bush, in an unusually fiery speech aimed at seizing the war-and-peace issue from the Democrats, asserted here today that presidential nominee Walter F. Mondale -- not President Reagan -- is the culprit behind the breakdown in arms talks with the Soviet Union.

"If you were a Soviet leader and you saw an election coming up in the United States between someone who you know is a tough negotiator, and someone else who has already promised before he sits down to cancel the MX, cancel the B1, and who has called for a nuclear freeze that would drastically lock in inferiority for our allies in Europe, and who would drastically curtail spending -- wouldn't you wait it out till Election Day?" Bush asked 2,500 students at the Student Political Forum at Texas A&M University here.

At a news conference afterward, Bush said he was not aware that Mondale had proposed an increase of about 4 percent in defense spending, adding that he will apoligize if he erred in speaking of drastic cuts. But he said Reagan's proposed increase is "drastically" bigger than Mondale's.

Bush, a generally reserved speaker, was uncommonly animated before the conservative student audience, which interrupted him more than a dozen times with cheers -- known as "Aggie whoops" in honor of their football team.

He hailed Reagan as a "decisive, strong principled leader," and denounced Mondale as a "handwringer" who calls the Sandinista leaders of Nicaragua "liberals" rather than "Marxist-Leninists."

Bush acknowledged later that Mondale had termed the Sandinistas "leftists," not "liberals." But he defended his statement, saying: "He doesn't want to call them Marxist-Leninists . . . . Maybe not liberals, I don't know. It's something less than what they are."

Bush also told the news conference that despite the remarks in his speech, he does not blame Mondale for the failed arms talks, but rather sought to show that "our record [on arms control] is a good record." The Russians will make significant concessions "once the Soviets realize they have [Reagan] to deal with," Bush said.

The speech marked an escalation in Bush's war of words on the Democrats, continuing his role as GOP point man on the war-and-peace issue.

In the past, he has called Mondale the favorite of the Kremlin and has charged that the Democrats do not "understand" the communist threat or the need for a strong defense. He also said Mondale is too "hot" for an arms-control agreement.

He accused the Democrats today of trying "to scare the American people about war and peace," and charged that "they don't even acknowledge that there is a threat from Marxist-Leninists" in Central America.

"We're not going to give up strategic weapons systems while the Soviets continue to build theirs up," Bush continued. "That's not negotiating; that's giving away the store. And this president simply is not going to engage in that."

Regarding the administration's "secret war" against the Sandinistas that Mondale has vowed to end, Bush held aloft a Nicaraguan stamp commemorating Karl Marx and declared to cheers:

"If you walk like a duck, and you quack like a duck, and say you're a duck, you're a duck. They're Marxist-Leninists. They are not liberals as Mondale says." It was this statement that he later modified.

Bush also continued his pattern of linking Mondale to former president Jimmy Carter. "Our opponents say that they don't think we can afford a growing economy and a strong defense," Bush said. "Maybe that's because under Carter and Mondale, we didn't have either."

Bush recalled the Iranian hostage crisis, and quoted one of Mondale's presidential primary opponents, Sen. Gary Hart (D-Colo.), in calling it "our days of shame." By contrast, he said Reagan had restored pride in the United States through such action as the Grenada invasion.

Aides to Bush remarked on his unusual fervor in the speech, during which he departed several times from his prepared text to embellish his attacks on Mondale. They attributed his tone largely to the enthusiasm of the A&M students, known for their love of country, Republicans and football.

Asked afterward if he has become the "tough guy" of the Republican national ticket, as Reagan takes the "high road," the normally mild-mannered vice president put his hand to his throat and asked incredulously: "Me? Oh please! I don't think so."