Sen. John Glenn (D-Ohio) is erect and reserved, from years in the military. Former vice president Walter F. Mondale is loose and outgoing, from years on the political circuit. They talk of President Reagan, the future, compassion and the Democratic Party, but they are different kinds of men with competing visions of America as they campaign across the country in pursuit of the Democratic presidential nomination.

Mondale has been called the candidate beholden to the special interests, Glenn the candidate who eschews those interests. But the more interesting difference is broader than that.

Mondale is the Democrat who goes into a union convention as a brother and who in Orlando, Fla., last week joked and bantered with the leaders of the Building and Construction Trades Department of the AFL-CIO as if they had been down that road many times before--which they had. The next day, in Maine, as he addressed meetings of party activists, he was, once again, familiar Fritz.

"I know the most about him," one woman said in explaining why she plans to support him.

Glenn is almost nonpolitical in his appeal, ending some of his speeches with an annotated recitation of the Pledge of Allegiance. His Democratic ties are much weaker. In Tacoma, Wash., recently, he butchered the name of the state Democratic chairman, a gaffe no one would allow Mondale to get away with.

On recent trips to Seattle, Mondale met with a labor group for breakfast, while Glenn addressed a gathering of engineers for dinner. "Finally," said one member of the audience as he approached Glenn after the speech, "someone who understands us exactly."

A campaign swing with Mondale is a trip through the Democratic Party; a similar swing with Glenn is a journey through Middle America. Along the West Coast recently, the airport crowds that turned out to meet the Ohio senator were composed largely of well-scrubbed youngsters and middle-aged couples, many with cameras and autograph books in hand.

The two men approach Reagan with a different view of the world. Mondale is harsh in his attacks, calling Reagan "a radical," "the most antiunion president in the modern history of America," and more. Fairness is a central part of his standard speech. "A president is supposed to teach us to be a caring and compassionate nation," he told an audience in Sanford, Maine. "This president is supposed to be someone who builds on that sense of community and caring. Instead of that, he is leading us into full retreat."

For Mondale, those attacks are a combination of conviction and political strategy. "Walter Mondale will be more liberal in the primaries and the general election than he sounds now," one of his advisers predicted. "If the economy is improving, the only way a Democrat wins is to polarize the election."

But Glenn and his advisers seem to sense something else about the country, which is why fairness is not a central part of the ex-astronaut's message. He talks of cuts in civil rights enforcement and the environment and some social programs, but after an uplifting defense of the New Deal as the vehicle that moved millions of Americans out of poverty, he quickly tells a Democratic audience in the state of Washington:

"Let me add one thing. I'm the first to admit that we overdid some of those programs, let them go a little too far, didn't really do what we should have done in watching how that national debt was building up."

Glenn is trying to present himself more generally as the candidate for the future, and his criticism of Reagan is not so much that the president is fundamentally wrong, but simply that he is a man who wants to return to a simpler past that doesn't exist. Which is why, once he has quickly covered defense, foreign policy, the economy and social programs, Glenn goes on at length on two subjects.

The first is education, and he comes across as a stern headmaster who would return rigor to the classrooms of America by setting goals. The other is, as he calls it, the need for "basic, seminal, Nobel laureate-type, breakthrough research." Glenn is most critical of the president as he describes what Reagan has done to federal support for research.

Mondale, too, presents himself as a man for the future, and also emphasizes the importance of education. And then he tells audiences: "The big question for you to ask is who among us will best prepare this country for our future and who's best equipped through experience and ability and issues and positions to lead this country on a sound and solid course."

He details his background in state government, in the Senate and as vice president, and adds, "I think I have the experience, the background, the commitment, the knowledge necessary to handle the toughest job on earth."

Glenn draws laughs when he plays off Mondale's assertion that the nomination should go to the person with the most experience. "He's going to be surprised when he learns that I agree completely with him."

And then he is off on a tour of his own resume: his nine years in the Senate, "enough to understand how Washington works;" his experience with an international business; his efforts to start four small businesses; his days in NASA when he worked with "the finest scientists in the country," and his 23 years in the Marines. He talks of next-of-kin letters written from overseas and says, "I don't need to watch late-night TV to understand what war is." No one, he says, "will work harder to keep the peace."

Glenn says that he is the man to lead this country into the 1990s, but what he really offers is stability.

"I don't think people right now want any huge new experiments in any direction," he said. "I don't know that we can really define what's normal in this country, but I think if there was a definition of it, people want to get back to what's more normal."

Critics call that warmed-over Reaganism, which is something Mondale has no intention of offering.

"My guess at this point," Mondale said last week as the campaign wore on, "there will be a debate about where this party ought to be going."