President Reagan, attempting to sweep all 50 states in Tuesday's election, paid a surprise last-minute visit today to Walter F. Mondale's home state, where he held his first news conference since July 24.

Although he clearly wanted to talk about his prospective landslide reelection victory, Reagan bowed to reporters' desire to discuss Latin America, taxes and one of his own gaffes.

Reagan acknowledged for the first time that he had made a mistake on Aug. 11 when he jokingly said during a radio sound test, "My fellow Americans, I am pleased to tell you I just signed legislation which outlaws Russia forever. We begin bombing in five minutes." But he said the press also was to blame for reporting the offhand remark.

The president also stopped short of ruling out a tax increase, but said he would attempt to seek new revenues by broadening the tax base rather than boosting rates.

He also said he would continue to seek U.S. aid for the CIA-backed guerrillas opposing the Sandinista government in Nicaragua as long as the Sandinistas assist leftist guerrillas opposing the government in El Salvador.

Reagan's strategists added Mondale's home state to the campaign itinerary this morning after a late-night meeting where they examined surveys showing Reagan trailing by 2 percentage points in Minnesota, and ahead in every other state.

"We just came from Milwaukee, and being that close, I just couldn't fly by Minnesota, particularly when I know our opponents are spending so much time in California," Reagan said in a statement opening the 25-minute news conference. " . . . Everyone knows that we've never written off any state nor taken any state for granted . . . . "

Reagan stopped here en route to rallies in St. Louis and suburban Chicago that climaxed three days of grueling campaigning in the Midwest. He then was scheduled to fly home to California, where he was to conclude his campaign Monday with three stops, including a rally at the state Capitol where he governed for eight years.

He also planned a paid nationally televised broadcast on all three networks Monday night.

Reporters expressed frustration at the president's inaccessibility throughout the campaign by delivering a barrage of questions. Appearing relaxed and cheerful, Reagan said:

* He wants no increase in individual income tax rates and has not seen any recommendations from Treasury Secretary Donald T. Regan on tax policy. The Washington Post reported last week that Regan would recommend a plan to end the federal tax deduction for state and local taxes, and to tax unemployment benefits and workers' compensation payments.

"I have no idea that anything of that kind will be recommended to me," Reagan said. He said he wants to obtain revenues from people presently avoiding taxes rather than get more from "those honestly paying their taxes."

* Today's election in Nicaragua is "a phony." He reiterated his support for the anti-government "contras," "because the Sandinista government is still supporting the guerrillas that are fighting against the duly elected government of El Salvador."

* The Soviet "bomb" joke, told at his California ranch during a warmup for his weekly radio broadcast, was just satire.

"You get tired, sometimes, counting to 10 as a voice check and so forth," he said.

"I was sitting in a small room ready to do my radio broadcast with a few of my own people around me -- and actually I meant it as a kind of a satirical blast against those who were trying to paint me as a warmonger," Reagan said. "So, having to do a sound check, I simply did it for the soundman's benefit."

Answering another question, Reagan said, "All right, I shouldn't have said it. But I further emphasize the media also shares in the responsibility for our national security. And I don't think they should have spread it. They weren't intended to hear it."

Reagan displayed some sensitivity about his lack of news conferences, saying that if the press counted the time he answered questions under the wing of Air Force One, it would amount to about six since Labor Day and "I think that that sort of belies the fact that I'm in a cocoon, and that I am not available to the media."

Reagan has held 26 news conferences in four years, fewer than any other modern president who served a full term. No other president has ever counted questions he answered while boarding his plane as part of a news conference.

At every rally this weekend, Reagan urged his audiences to "win one for the Gipper," a reference to the legendary Notre Dame football player Reagan portrayed in the movie "Knute Rockne -- All American."

Today, in Rochester, Reagan was asked in reference to his Minnesota visit whether the Gipper would "run up the score."

"I don't think of it as running up a score," he said. "The Gipper would never quit before the final whistle."