As Walter Mondale and his traveling circus vacated the Century Plaza Hotel for one last rally in Mason City, Iowa, en route to Minnesota, President Reagan and his traveling circus were moving into the hotel tonight.

Reagan spoke to the nation via television, the medium that has always been kind to him, asking for help "to complete the task we began together." Mondale, trying to put the best face on his long-shot quest for the White House, roared to the thousands who turned out at his final California rally, "We can make history by giving them the biggest upset ever!" The noon Pershing Square rally, though, seemed to fall short of its goal of drawing 40,000.

In these final days of what has seemed an endless campaign, Reagan and Mondale careened in and out of airports and rallies from Boston to Los Angeles. At times, in the heartland of America, they were but hours behind one another. While Mondale spoke to Hispanic Americans in the Rio Grande, Reagan took a shot at Mondale's home state of Minnesota, where Mondale has but a slight lead in most polls.

Yes, they both saw America, but they saw different nations. Mondale, the anointed loser in the polls, was pleading for an "America of fairness," speaking to -- and for -- the have-nots, whatever remained of FDR's Democratic coalition. Reagan was speaking to the larger percentage of Americans who see themselves as better off -- overwhelmingly white, the working- and middle-class as well as board room directors. The only thing he has to fear is complacency itself and so the president exhorted one and all to vote. But for all Reagan's acting skills, he was less than convincing when he said, with a little half smile, "Those polls just scare the life out of me."

Nor did Mondale's smile always come easily when he was introduced as the man who is going to score the "biggest upset victory in the history of the United States."

These two men, the trailing challenger and the incumbent, are divided by much besides polls: perceptions of their country, the role of government. The contrast in their messages is stark. Yet, unlike 1980 -- when Jimmy Carter retreated into cloistered desperation and Reagan loped calmly as the front-runner -- both candidates are upbeat and cheered by masses of the adoring and faithful.

Ben Travino stood in the muggy heat of the McAllen Civic Center in McAllen, Tex., holding his young daughter's hand. "We don't listen to polls," said Travino, with a heavy Spanish accent. "Polls are for people who have everything. For people who have telephones. A lot of people don't have telephones down here.

"I brought my daughter out to see Walter Mondale. He is the last of a dying breed," said Travino yesterday. "The breed that cares for the people. Reagan should come down to the barrio where we live."

Travino knew intuitively what political observers are saying. In four years, the Democrats will field candidates who probably won't come within miles of the Rio Grande, or black Baptist churches like the one in Memphis that Mondale visited yesterday morning. The new breed -- the "Atari Democrats" -- will be seeking to reach the independents and the young who seem untouched by the old liberal chords.

Had the race been closer, Mondale might have trimmed his sails as well. These past weeks, however, Mondale made the decision to go to his strength. His private talk has an air of resignation as he says he is speaking for the "core of a decent society" and has "no regrets at all." He seems determined to win a "moral victory." In so doing, he has ceased being cautious and become more appealing, blasting Reagan as lacking compassion, buoyed by the thousands who cheer. He seems driven now by the hope of at least closing the gap, of not dragging statewide Democratic candidates down.

Travino applauded wildly with everyone else in the appreciative Rio Grande crowd when Walter Mondale's son, Bill, introduced his father in fluent, Gatling-gun Spanish. The heat was too much even for Mondale, standing there in his "full Norwegian" -- dark suit, white shirt and maroon tie. The thought occurred that he could get a couple of hundred votes if just once he wore a bright shirt. Mondale took off his coat, rolled up his sleeves, leaned on the podium and spoke with a kind of intimacy that was absent a few months ago:

"The suffering in this valley is building in the thousands and this is enough to make a stone cry." Texas Sen. Lloyd Bentsen had just referred to Reagan as the "man who came here recently and said, 'People live not by bread alone' and then went to work and vetoed the health bill for the Rio Grande valley." Mondale was cheered loudly as he speaks to their pride. "The issue is not what America can do for Hispanic Americans. That's never been the issue. The issue is what Hispanic Americans could do for all Americans if we would open those doors. I know your gifts. I know your talents. Your love for your country."

Later, a stadium in Corpus Christi was packed with some 20,000 Hispanic-Americans. There was a feel of the past as Jose Feliciano sang "Blowin' in the Wind" and "Light My Fire," changing the words -- "Mondale's going to light our fire." An airplane carried a hopeful message trailing in streamers: "Truman, '48, Mondale, '84."

Mondale spoke of administration broken promises: "They haven't done one single thing for this community and you know it," he shouted. The stadium roared with applause when he said, "Well, for four years they did it to you. On Tuesday you can do it to them!" Scenes

A hangar in Tri City Airport, Saginaw, Mich., looked like a mini-replica of the Republican convention. No fewer than 20 flags stood behind Reagan and two huge Pattonesque flags ran from the ceiling to the ground. Nets hung from the ceiling, swollen with balloons. Bands played furiously, Michiganders bundled in mufflers and hats stomped their feet against the chill. "My opponent may take a negative view of America, but he better not try to peddle his doom and gloom in Michigan!" said Reagan into the cold, predicting "better days to come."

Michigan employment has returned only to pre-198l-recession level, but perception is what matters. Many in the state have been down so long that it seems like a real up to them. In one Democratic working-class district, about 3,000 stood in the rain for Reagan. They cheered as do all of Reagan's crowds when he said, "Ours is the land of the free because it is the home of the brave."

Reagan took credit for everything -- from crime going down to SAT scores going up. Democrats in the audience eagerly told why they are switching. One housewife liked him for "his stand on abortion and for not raising taxes." A man next to her said, "When President Reagan took office I was laid off and so were quite a few of my friends. Now we're all working." In the simplest of terms, Reagan urged them to vote for local Republicans who are "all against higher taxes and for strong defense."

Anti-communist fervor abounds at Reagan rallies. Roars drown him out when he says, "During these last four years, not one inch of territory has been lost to communist aggression. And the United States is more secure than we were four years ago. Yet my opponent sees a different world . . . One year ago, we liberated Grenada from communist thugs . . . My opponent called what we did a violation of international law that erodes our moral authority to criticize the Soviets." "BOOOO!!" shouts the crowd. "Well," Reagan says, "there is nothing immoral about rescuing American students whose lives are in danger!"

Teen-age faces light up when he tells them, "Your generation really sparkles. Your idealism and love of country are unsurpassed." Many of the young men are vague about fighting themselves, but thrill to "standing tall" rhetoric. Kevin Holmquist, 18, a Syracuse University student loaded with Reagan buttons, says the campus is for Reagan, 3-to-1. "I hate communists," he says, between pops of his bubble gum. "The liberals want to pull out advisers and give aid to the, uh, costras. Well, are we going to let Cuba take over?"

Meanwhile, Mondale says he wants to strengthen the Peace Corps as well as the Marine Corps. In Rock Island, Ill., he shouts to a rally of farmers and laid-off workers from International Harvester and John Deere: "Does anybody here need that illegal war in Nicaragua?" "NOOO!" the crowd shouts back. "What we need is human rights and justice. We need a president," he says, who will negotiate to stop "those God-awful nuclear weapons or there will be no future at all."

Both Mondale and Reagan deliver a standard stump speech. Despite his nasal voice, Mondale effectively delivers his without notes, in a personal manner. Reagan is still a master of canned one-liners, delivering them as if they were ad-libs; but, surprisingly, he continues to read from a text that is known by rote to many who cover him. A sure winner is to say of Mondale, "If his program were a Broadway show, it would be 'Promises, Promises.' And if it were a book, you'd have to read it from the back to the front to get a happy ending." When Reagan goofed and said at a rally in Springfield, Ill., that "you'd have to read it from the front to the back," a press chorus shouted the right phrase for him.

Both camps boost the crowd estimates to ludicrous levels. Reagan's hangar rally -- which could not have held more than 4,000 -- was billed as a 14,000 winner. Mondale touts his crowds in superlatives -- "the biggest ever in the history of -- -- ." The signs of hecklers follow both campaigns: "Shut up Mondull," "Reagan 50, Mondale 0" greet Mondale; Reagan gets "No more 'Ray-gun' " or a clock superimposed over his face, saying, "Bombing will begin in five minutes." A steady diet of either camp's rallies could put one in danger of balloon poisoning. Up Close

"We land, we arrive, we listen, we leave," sighed one reporter covering Reagan. It is an easier life than the Mondale campaign. There is spoon-fed, around-the-clock photocopying of the stump speech, corrected transcripts, cosseting by the staff and an early bedtime. But not much goes on.

For months the frustrated press corps has been writing about the "cocoon candidate." Although a pool rides on Air Force One, Reagan has never come back to where they sit, in steerage. Occasionally he gets as far as the Secret Service compartment, and waves. Reporters who trail on the other two planes send up questions.

For months, the only hope was to stand under the wing of a usually roaring plane and shout questions. Which generally led to reports saying that they couldn't hear much of the president's answers. His press conference Sunday was his first since July. Staff aides counted up all those two-minute shouting forays and said they added up to four hours.

Meanwhile, the Mondale press was reeling into California after a day that had started 12 hours before. Everyone was suffering from The Blur, which, in one account, includes "a tendency to follow lines of people almost anywhere, a tendency to stop wherever people have gathered and take notes, wild mood swings whenever the topic of laundry is raised . . . You know when you've been on the campaign trail too long when . . . you begin to think that most cities in America are tinted the same color as the bus window." Author of this report, "The Plane Truth," is Dayton R. Duncan, Mondale's deputy press secretary.

After two days with either campaign, clothes don't fit. Force-fed like geese on every hop, no one seems able to pass up the equivalent of nine meals a day that can go something like this: eggs Benedict, orange juice, coffee, Danish, bloody Mary, cheese, crackers, fruit, hoagies, pizza, guacamole, tacos, steak or lobster, roast beef sandwich, pasta, rice, artichokes, lox, bagels, crab claws, shrimp, more steak, wine, beer, assorted booze, Snickers, peanuts, ice cream, salad, cake. When Mondale talks about a compassionate administration that could airlift food over to the starving Ethiopians, guilt-ridden journalists offer to airlift the leftovers.

At one time in the long ago, before the Iowa caucus, front-runner Mondale also played the Imperial Candidate. No longer. In mock imitation of the crowds that shout "We want Fritz," a gaggle of reporters can often get him to stop for mini-press conferences at airports. Taxes

Mondale gambled when he said at the Democratic convention that he -- and Reagan -- would raise taxes, but that Reagan "won't tell you until after the election." In the last hours of this campaign Mondale seized on a news report as evidence that the president plans to raise taxes unfairly -- to which the president replied, "Over my dead body."

The report -- that the Treasury Department has given tentative approval to three new revenue sources -- became a centerpiece of every Mondale speech and impromptu press conference. Mondale said they would tax unemployment benefits and workers' compensation -- "If you get sick or you get ill or damaged or mangled on your jobs, those payments that go to keep you alive, you'll pay taxes on that." Deductions for taxes paid to state and local governments would also be eliminated, he said: "We've never done that before in American history -- that's called a double tax, a tax on a tax." Although some economists predict that the economic recovery is temporary, and say the deficit is dangerous, Mondale's frustration is that he preaches only to his converted.

Reagan, on the other hand, gets applause when he quotes Mondale as saying he has consistently supported legislation "time after time which increases taxes on my own constituents." Reagan pauses. "Now doesn't that make you want to be one of his constituents?" Laughter always follows his line that parodies Mondale's tax plan. "He's got an economic plan and it has two parts -- one, raise taxes; two, raise them again."

When told that Reagan said the Treasury Department report was "absolutely untrue," Mondale shot back, "Let me give it to you raw. There is no question in my mind that they're planning a tax increase after this election but they haven't had the respect for the American people to tell them what's in it. I have no doubt it will strike at middle- and moderate-income Americans just as everything else Reagan has done in the last 20 years. If he has a different proposal, let's hear it. He's demagogued my tax position all over this country. He can live with it now." Church

The small church in Memphis has its memories of Martin Luther King Jr. He spoke there and was on his way to supper at the home of the Rev. Samuel (Billy) Kyles when he was shot. Yesterday morning, "the Right Rev. Mondale," as Kyles called him, preached a political sermon there. Kyles said, "Maybe Mr. Reagan don't want to go to church because the Holy Ghost might come on him and make him do right."

Mondale sounded the themes he has for several days now. He spoke of the world seen in Reagan commercials: "It's all picket fences and puppy dogs." And: "When was the last time you heard the word decency, the word justice, compassion at the White House?" The people who crowded the pews murmured "Amen" and "Yeah, yeah." At another church, Mondale had said, "No one's hurting. No one's alone. No one's hungry. No one's unemployed. No one gets old. Everybody's happy."

Leaning on the pulpit, Mondale said 4 million children had been taken off the school lunch program. Of the Reagan administration, he said that "this crowd's record is an insult to American decency. This crowd can hear the faintest drum -- but they can't hear the cry of a hungry child." He spoke of unemployed black youth: "The deadliest of all sins is the mutilation of a child's spirit." He spoke of the disabled, "the mentally ill, the handicapped, the broken, the aimless, the dispirited, those sad women who walk the streets, those people who sleep over grates. They took a computer and they terminated a half million from disability roles. Just turned them out on the streets. No way to defend themselves. Some of them died. Some of them committed suicide, all of them are in desperate shape."

It is a "crucial," "watershed election," says Mondale, saying the administration "trashed" the Civil Right Commission, warning of possible Supreme Court appointments, and saying that he, not Reagan, will fight discrimination. The Finale

To everyone who believes the polls, Mondale's last hurrah came today, but so, in a way, did Reagan's. This is his last political race, and although Mondale is not talking about his future few imagine him trying for the presidency again. So their final battle takes on a special meaning as they reach out to their separate Americas. For Mondale, there was perhaps an unintentional poignance when he spoke to the rally here and said, "Tomorrow someone is going to make history."

To his followers he added, "Let it be us. Let it be us."