Mary Ellen Chamberlain lives in a clapboard house standing above the Mississippi River, where Huck Finn was the embodiment of one American dream. This is where presidential politicians now come to begin their climb toward another.

Just off the dining room Chamberlain has turned a small alcove into a most unusual trophy room.

It is full of memorabilia, and Jimmy Carter smiles from every wall. His teeth gleam from autographed photographs that capture him in the wintry plains, freeze-framing the 1976 Democratic candidate in the nearby living room pumping the hands of Chamberlain's friends and neighbors and immortalizing him in the Oval Office to which they helped propel him.

Carter went to the house five times in his presidential quest, and the simple home symbolizes the symbiotic relationship between Iowa and Carter, each moving the other irrevocably into the political limelight.

So now Sen. Gary Hart (D-Colo.), a presidential aspirant who registers as a single digit in the national polls--as did Carter at this point seven years ago--is in the dining room, pumping hands and asking Iowans to do for him what they did for the Georgian.

It is late January, two years before the inauguration of the next president of the United States, more than a year before Iowa's first-in-the-nation Democratic caucuses.

Isn't that a bit early, even in this modern era of presidential-election marathons?

According to Democratic politicians in Iowa, Hart, if anything, was a little slow getting out of the blocks for a nuts-and-bolts candidate whose hope is to organize a blitz of the two small states--Iowa and New Hampshire--that provide the early momentum in a presidential campaign.

He apparently was not too early in the troubled towns of a worried country. Even for a dark horse, Hart's crowds were large; they were filled with hurting farmers, angrily unemployed laborers, and eager liberals looking for a way back into the action.

Nor did Hart suffer from the loneliness of the long-distance runner. Trailing him were a half-dozen Washington-based reporters from major newspapers, news magazines and television networks.

Nor did it seem early in the two states that start it all.

In the lobby of the Savory Hotel in downtown Des Moines, the Democrats' hangout, the labor leaders, precinct pols and political junkies already have staked out their seats and wait impatiently for the candidates to pay them homage and the network cameras to record their wisdom.

Sen. Alan Cranston (D-Calif.) has formally announced that he is a candidate for the 1984 Democratic presidential nomination, and Hart, former vice president Walter F. Mondale, the front-runner, and former Florida governor Reubin Askew are to make their announcements later this month.

Mondale has paid 20 visits to Iowa since Labor Day last year. The pols grumble that Sen. John Glenn (D-Ohio), the pollsters' second choice, seems to be lost in orbit and wonder when he will come to Earth to press the flesh in the Savory. Time is running short, even for a national hero.

In Chamberlain's living room, the senator from Colorado, 6-foot-3 from the soles of his black cowboy boots to the top of his brown hair, spins off honed coffee-klatch phrases.

" . . . You are caught in the massive failure of a reckless and unwise Republican economic experiment," Hart says.

The troubled farmers break into applause, sending cigarette ashes onto Chamberlain's well-trod rugs.

" . . . People are eating out of garbage dumps in Phoenix . . . . People are living in boxes and tents . . . . Fifty years ago, when the nation found itself in the same kind of economic disaster, it turned to dramatic leadership . . . ."

The Iowans break into applause again, sending fumbled canapes and crumbling cookies to the floor, where Chamberlain's little gray dog darts from under the table like an automatic vacuum cleaner programmed for this clean-up chore.

" . . . I will invite the Soviet leader--challenge him, if need be--to meet with me and deal with the nuclear arms race the day after I move into the Oval Office . . . ."

Loud applause. Then Hart is leaving, pumping hands through the kitchen, leaving behind the inevitable sign-up sheets for volunteers.

It is late, almost 8 p.m. of a long day that still isn't over. In the driveway the van that has taken him more than 200 miles across browned prairies sits warming.

The senator, his wife, Lee, and a handful of reporters pile back in for the drive to Mount Joy airport where, in the dark, the only waiting plane is Hart's chartered six-seater.

The plane bucks easily through a light Iowa snowstorm on the 45-minute flight to Des Moines, landing an hour late. One more meeting tonight, a 7:30 breakfast in the morning.

"Anybody dumb enough to want to be president," Hart says with his half-distant smile, "is too dumb to be president."

Then he disappears into the night. His wife says she knew he was ready last year when he told her he wants the presidency more than anything in the world.

In the next seven-day period Hart drops quickly back into Washington, heads south to Florida, does a quick pass-through in New York, crams a full day into a half-day in Massachusetts and then spends two days barnstorming in the first primary state, New Hampshire.

Hart, 45, is making his first run for the presidency, but he has been through all this before as the boy-wonder campaign manager for Sen. George McGovern (D-S.D.) in 1972. It is difficult to assess its political value now. Hart tends to play down his role, casually lumping it on the same level as the college-boy door-bell ringing he did for John F. Kennedy in 1960.

But he collects old McGovernites at every stop, just as he collects Kennedyites who are suddenly without a candidate, and disenchanted party regulars who see Mondale as a reflection of the Carter administration.

Hart is an unusual candidate, issue-oriented but almost too mechanical. He is an organization man, following lessons learned in the orchestration of the McGovern campaign. It is a talent that makes it impossible for the front-runners to ignore him in small states like Iowa, where perhaps 75,000 caucusing Democrats may provide the first surge of momentum for the probable winner in 1984.

"There are three strategies for winning a campaign," Hart says, his chiseled features turning rigid and serious as the van cruises Iowa. "Endorsements, media and organization." He stares out the window. "I've chosen the third. It's the only one I know how to do."

Still, Hart also has tried to stay a half-step ahead of the other candidates on issues, a key to attracting activists but a political danger that has brought down some other organizational champions. He talks about laying out a national strategy for modernizing American industry and moving rapidly to reduce the arms race.

In New Hampshire, making a play for the activists, he issued a statement criticizing President Reagan for failing to mention acid rain in his State of the Union address.

More conventional issues, such as his stand on free trade, sometimes bring him grief, as with Iowa auto workers who want protective legislation against Japanese imports.

In a Dubuque meeting with United Auto Workers leaders, away from which Hart campaigners hurriedly hustled reporters when the going got rough, the senator met only steely eyes and unrelenting jutted jaws when he told them the answer to their plight was plant modernization, not protective legislation.

Later, when one of the union leaders was told by a reporter that most of the other Democratic candidates were "holding their noses" as they supported the auto workers' position, his response was blunt and terse: "We've had to hold our nose when we went into the voting booth a few times, too."

Hart is an intense, inward man. Years ago, during the McGovern days, he once told an interviewer, "I never reveal myself or who I really am."

He also is driven, as he would have to be to endure two years of 7 a.m. breakfasts in Leney's 10-stool working-man's cafe in Manchester, N.H., and late, light-plane flights through snowstorms for a long-shot chance at Chamberlain's trophy room and other honors.

In 1972 he was asked about the effect of the campaign on the rest of his life and his marriage.

"I have no personal life at all," he replied. "I lead a completely political existence. If one person doesn't share the same interest, you've got a problem. Let's just say I believe in reform marriage."

A decade later, in 1982, Hart and his wife separated, and he began the long quest for the presidency alone. But Lee Hart, an outgoing woman with far more flair for the offhand comment than her husband, was back with him on the campaign trail a few days ago in Iowa.

The klieg-lights reconciliation produced a few difficult moments. At one reception in Waterloo, Hart forgot to introduce her, then awkwardly acknowledged the omission.

At another, he was swinging lightly through a campaign joke about the down-home job of collecting caucus workers. He said he would bring his snow shovel on his next trip and do odd jobs for supporters.

"In the winter I do driveways and sidewalks," he said jovially. "In the spring, I do windows and light housework."

The room rippled with friendly laughter. Then a voice cut in abruptly from the audience:

"You might start practicing in Washington."

It was Lee Hart. For a moment the room was silent, and Hart stood wordless, his face flushed with uncertainty. Then the crowd roared and the senator laughed uneasily.

When the Hart team reached New Hampshire five days later, Lee Hart was introduced without fail, the light-housework line was in all the speeches--and her no-longer-spontaneous voice from the crowd was part of the repertoire.

After all, there isn't that much time--only 24 months until the inauguration--and in the presidential marathon you capitalize on every good one-liner that comes to hand.