Walter F. Mondale today pledged, if elected, to seek a summit meeting with the Soviet Union within six months of his inauguration to negotiate a nuclear freeze, as he and running mate Geraldine A. Ferraro declared the fall election to be a referendum on war and peace.

Standing in a chilly drizzle before an enthusiastic crowd of 2,500, Mondale pounded the rostrum and shook his fists as he declared, "In this election there's much at stake, but nothing's more important than getting control of those weapons and moving toward a safer world."

President Reagan "has a thing about arms control. He thinks it's weakness," Mondale asserted after peeling off his jacket, loosening his tie and rolling up his shirtsleeves despite the rain. "I will fight for arms control on the first day that I am in the White House."

Mondale's attack today reflected his belief that Reagan is most vulnerable politically on the issue of arms control, where talks between the Soviet Union and the administration have broken down.

Under the plan he outlined today, Mondale on his first day in office would ask Soviet leaders to meet with him in Geneva to begin "substantive negotiations" toward a nuclear arms freeze.

As an incentive for the Soviets to join those talks, Mondale at the same time would unilaterally declare "temporary moratoriums" on testing of all nuclear weapons and testing and deployment of all space weapons.

In a 20-minute speech to the downtown rally before Mondale spoke, Ferraro denounced Reagan for failing to achieve "a single diplomatic success in 3 1/2 years."

The New York House member sharply rebuked her Republican counterpart, Vice President Bush, for claiming in Texas on Tuesday that Mondale "would drastically cut" defense spending. Mondale advocates defense increases but at a more moderate rate than Reagan wants.

Bush also implied that Mondale was responsible for the Soviet refusal to return to the bargaining table before the Nov. 6 election in hopes of cutting a better deal with the Democrat.

"Now that's a good one," Ferraro said. "Mr. Reagan is the first president since Herbert Hoover not to sit down with his counterpart from the Soviet Union. He's the first president since the dawn of the nuclear age not to enter into an arms control agreement."

Bush, campaigning in Kentucky, mocked Mondale's pledge to seek early talks with the Soviets. "Let him try," Bush said, adding that Soviet leaders "are not ready to talk."

Ferraro portrayed Mondale today as an informed advocate of arms control since at least 1969. She contrasted her running mate and Reagan by suggesting that the president fails to grasp not only the importance of arms control but the regional factors behind conflicts in such places as El Salvador and Lebanon.

When the Soviets negotiate with Mondale, Ferraro said, "they'll have to deal with a man who understands the world and knows what he's doing. They'll face a president who has both the capacity and the commitment to negotiate serious arms control."

Ferraro used harsher language than usual in blaming the administration for last year's bombing of the Marine barracks in Beirut, Lebanon. "Our leaders made a mistake and over 250 young men paid for it with their lives, she said. "It was horrible when those men died, but it was an outrage that they died for no reason."

The Democratic candidates were reunited here in the culmination of a swing that began Labor Day and chased Reagan -- with his commanding lead in the polls -- to California. Mondale began today in Salt Lake City, where he listed the details of his arms control proposals in a speech to the 66th annual convention of the American Legion.

Reagan addressed the legionnaires Tuesday, and Mondale sought to counter the president's claims that he resurrected military strength from years of decline under President Jimmy Carter. "Every day that we let the arms race continue, more nuclear warheads are aimed at our people and our allies," Mondale said. "Every day we fail to open negotiations with the Soviets is another day we slip toward Armageddon."

Wearing his blue-and-gold legionnaire's cap as a member of Sanford Post 192 in Elmore, Minn., Mondale affirmed his patriotism and commitment to "steady, sustainable growth in military power."

He questioned Reagan's claimed "desire for a world without war."

"I do not challenge the sincerity of that desire," Mondale said. "The issue is whether he has set us on a course toward peace and a safer world, and in my judgment he has not."

"He has conducted an arms race on earth, and now he wants to extend it into the heavens," Mondale said of Reagan. "He even makes jokes about nuclear war. It's not funny."

Mondale aides said his temporary moratoriums on nuclear and space weapons would last at least six months and perhaps longer, if progress could be made in negotiations.

Aides said they could be broadened to include other weapons and become a veritable "quick freeze" as advocated by some freeze activists. But, the aides cautioned, they were meant only to break the currrent "logjam" over arms negotiations and not as a substitute a mutually verifiable nuclear freeze.

In today's speech to the veterans, hastily added to Mondale's schedule, his strategists sought to offer a virtual back-to-back comparison with Reagan on issues.

Mondale contended that the nation is "dangerously dependent on nuclear weapons," that its alliances overseas have grown "flabby," and that conventional American forces are "not what they must be."

"A trillion dollars spent on defense has not given us a trillion dollars of military preparedness," Mondale said.

If elected, he said, he would press allies in Europe and the Pacific to pay a larger share for collective conventional defenses, and would "make a fundamental shift of resources toward readiness and sustainabiliy of conventional forces."

He said he would place an emphasis on "survivable strategic forces," those considered more likely to withstand a nuclear strike.