Walter F. Mondale, pounding his fists and speaking emotionally, today discussed his values in detail and appealed for voters' trust as he visited farm towns of his native Midwest.

"I ask you to think it through," he implored farmers in a barn in Quincy, Ill., just after sunrise at the start of a 115-mile bus trip into Iowa, Missouri and Wisconsin.

"See what you've seen. Ask who you trust. And then by the hundreds, and then by the thousands, and then by the millions, walk into that polling booth and take control of your government," Mondale said.

Just as President Reagan did on a whistle-stop Farm Belt tour two weeks ago, the Democratic presidential nominee borrowed heavily from former president Harry S Truman. But Mondale was able to draw more effectively on Truman's partisan zingers and his familiar lesson about underdog victories.

Truman, he recalled, once delivered a farm speech on an improvised podium of two-by-fours atop a manure spreader.

"Old Harry said, 'I've spoken all over the world, but this is the first time I've ever spoken on a Republican platform,' " Mondale recounted.

The bus trip went as if Norman Rockwell had been its advance man. Mondale's caravan chugged past browning corn stalks and towering grain elevators and was greeted on each sun-dappled main street by flag-waving school children, snappily attired marching bands, baby-toting mothers and toothpick-chewing farmers.

The candidate seemed lifted by crowds and scenery and, at each stop, drew on his rural roots.

"I am the only pea-lice inspector ever to run for president," he told Quincy farmers.

"I never lived in a community this big," he said in Canton, Mo. (pop. 2,000). "Our town was 900, if you counted the dogs."

In Keokuk, Iowa, he said: "I am your neighbor. I grew up, as you did, learning the values of the Midwest. I was taught you can make a mistake and be forgiven. But if you're dishonest, that's not tolerable.

"I was taught the importance of work and . . . of dignity and the love of our country and the belief in our faith. I was taught that public service is as high a calling as possible," he said.

In Burlington, Mondale said he could "smell victory in the air," and at other stops he recalled Truman's 1948 campaign.

"I rode on the Truman train, even though I was too young to vote," he said. "And in that campaign they wrote Harry Truman off. Couldn't win. It was hopeless. They stopped taking polls, he was so far behind.

"But they forgot something. Out in every one of those little farm towns, every one of those little stores on Main Street, every one of those churches, people were thinking. They weren't hollering or making noise. Each of them was asking, 'Wait a minute now, we can't continue this way. We need to strengthen rural America,' " he said.

For nearly two generations, political underdogs have been encouraged by Truman's startling upset of Thomas E. Dewey, and Mondale's campaign is no exception.

With televised debates behind them, their arguments before the public and gaps in the polls still daunting, candidate and staff know that the prospect of victory is remote. Yet no one has visibly lost spirit or hope.

"We've been schlepping around the country for two years now, and these last few weeks are the first time it's really felt like a presidential campaign," speech writer Martin Kaplan said.

Although he trails in each farm state, Mondale believes that foreclosures and low commodity prices in the Mississippi Valley make this fertile political terrain. He told farmers that he opposed President Jimmy Carter's Soviet grain embargo, and he attacked Reagan for trying to blame failed farm policies on someone else.

But most of today's oratory was uplifting as Mondale cited his commitment to education, the environment, civil rights, social justice and arms control. He also made one farm-policy speech in Quincy, where he pledged to manage farm programs in a way that would keep supply in line with demand, to put a six-month moratorium on Farmers Home Administration loan foreclosures, to "get tough on trade" and reject farm embargoes, to strengthen rural electrification programs and to feed starving people everywhere with America's agricultural abundance.