In the final week of his presidential campaign, believing he has the electorate's ear, Walter F. Mondale will hammer point by point at his differences with President Reagan, his staff said today.

"It's thrilling, in a way," said his press secretary, Maxine Isaacs. "He's got the attention of the American people and he's talking every day about what he wants to talk about. How many people have had that chance? How many have run for president -- 80 or 90?"

Isaacs and other aides said Mondale is not engaging in a lose-with-honor approach, as some have speculated. They said that they believe their differences with Reagan are so fundamental and cut across such a wide variety of issues that his best hope lies in offering a broad reprise of his policy positions.

Isaacs said Mondale has decided to pick an issue a day. "Each night he tells us what he wants to focus on the next day," she said.

Mondale returned today to deficit reduction and tax fairness -- an issue he pushed in September, then largely abandoned after polls showed it was not cutting with the voters.

On Saturday, he spoke about the Supreme Court, civil rights and the separation of church and state, also issues that had been a focal point earlier in the campaign. He next was expected to deliver a speech about human rights, and later in the week, there is to be a farm speech and a speech about the obligation of government to the poor, the elderly and the handicapped, Isaacs said.

Mondale has not made arms control the principal focus of any of his speeches in the past week, as many expected he would. However, a five-minute television ad that aired on all three major networks tonight featured shots of young children intercut with shots of exploding bombs. "We know that if the bombs go off, that's probably the end. It's over. We're the first generation that has the capacity to destroy all life," Mondale says into the camera.

Whether the strategy can erase Mondale's large and growing deficit in the polls is uncertain, but it clearly has done one thing -- produce a candidate who is as loose and relaxed as he has been all year.

During a leisurely morning in San Francisco today, Mondale took an unscheduled walk from his suite at the St. Francis Hotel to the front of the Moscone Center, where he won his party's nomination at this summer's convention. Along the way he posed for pictures with pedestrians and helped a bellboy move luggage.

Later, at a news conference outside the Calvary Presbyterian Church in San Francisco, where he had attended services, Mondale answered a question about why he is optimistic by motioning to the hundred or so local residents who had gathered for the event, encouraging their applause. They obliged, Mondale smiled.

Moments later, Mondale described Reagan's approach to tax policies. "He says that the income tax was shaped by Karl Marx and the flat tax was endorsed by God," Mondale said.

In a speech here tonight, Mondale said, "It comes down to this. Anyone making $70,000 or less a year -- that's nine out of 10 Americans -- will do better under Mondale than under Reagan."

"Mr. Reagan has been saying that my plan is based on envy and his on opportunity. I say that my plan is based on fairness, and his on greed. I trust the American people to tell the difference."

Mondale said that one of the biggest winners from Reagan's 1981 tax cuts was his former employer, General Electric. Over the past three years, he said, GE earned $6.5 billion in profits, paid no federal income tax and has claimed tax refunds of $283 million.

Mondale also said tonight that the Republicans were taking the voters "for granted. The president is sleeping at Camp David today. They've got old George Bush hidden in the basement. And I'm out here with the people."

From here, Mondale will travel to Washington, Minnesota, Illinois, Kentucky, Maryland, New York, Ohio, Massachusetts and Pennsylvania over the next five days. Over the weekend he is expected to head west again, via Texas, to stage a rally in Los Angeles on Nov. 5, the day before the election.