Walter F. Mondale today made a passionate plea to heartland Democrats and fellow midwesterners to repay him for a lifetime of service to them and give him "the biggest upset this country's ever seen in a long, long time."

"This election is up to you," he told 2,000 wind-buffeted onlookers this morning in the town square of Mount Clemens, Mich., outside Detroit. "I've traveled across this state hundreds of times over the years. Every time Michigan has asked for help, I've said yes.

"Every time the workers of this state have needed help, I've been there. When your kids have needed an education, I've helped. When you've needed trade-adjustment assistance, I've been there. I love this state. We're fellow midwesterners. Give me this chance to be your president."

At the Rock Island County Fairgrounds here in East Moline, Mondale urged farmers to "raise less corn and more hell next Tuesday," a suggestion that was greeted with a rousing roar of clanking cowbells, bleacher-stomping and wild applause by about 4,000 persons in the grandstand.

In a pointed attack on President Reagan, Mondale said that when it comes to the needs of farmers, "The president of the United States who's in office today doesn't have a clue to what's going on out here."

Mondale also continued his attacks on the administration's tentative plans to propose taxing all unemployment and workmen's compensation benefits and to eliminate tax federal income-tax deductions for state and local taxes.

Eliminating the state and local tax deduction, he said, would amount to a "tax on a tax." As for taxing unemployment and workmen's compensation, he said, "Let him try unemployment and see how they like it."

Three days before voting begins, enthusiastic crowds are about the only sign of the groundswell of support that Mondale strategists have hoped would push him into a closer race with Reagan.

The final Washington Post-ABC News survey of 48 states showed Mondale hardly within striking distance in enough states to capture the necessary 270 electoral votes for victory.

With the South all but eliminated from his would-be victory formula, Mondale needs a near sweep in the arc of large industrial and agricultural states stretching from New York around the Great Lakes to his native Minnesota.

Even with that, Mondale must win in border states and almost certainly must capture Texas, which every winning Democrat has done in this century, and California, where Reagan has never lost an election.

On Sunday Mondale is to campaign in Tennessee and head to Texas, states where increased voter turnout among Hispanics and blacks are essential to his effort. Sunday night he is to head to Los Angeles for a final West Coast appearance.

Today, the weekday crowds that for so long had buoyed the Mondale campaign thinned in number but not in vigor. Many of those who stayed to greet Mondale after his appearance in Mount Clemens offered words of encouragement.

"Go get 'em, Fritz," said one. "You're gonna make it. Harry Truman did," said another.

"We will; we will," Mondale resonded.

Here in East Moline, Mondale was ebullient in tone and seemed so in spirit. He joined in on a hearty "boo" at the mention of Vice President Bush.

Mondale held a news conference in St. Louis, attended a rally in Fairview Heights, Ill., and was headed for a rally in Memphis.