The contest between Ronald Reagan and Walter F. Mondale was from the beginning one between a man and a party. Nowhere was the point made more vividly than in the Democratic city of Boston in the final hours of the campaign.

The two chased each other through the most political of American cities a day apart just before the curtain went down. Reagan demonstrated his personal wallop; Mondale the strength of the party that nominated him.

Reagan spoke at a rally in the City Hall Plaza, and he was the whole show.

Mondale, 25 hours later, on the Boston Common, was another speaker at a gathering that illustrated the verve and variety of the Democratic organization, although not necessarily its chances for success even in this heavily Democratic state.

The weather spoke of their comparative luck.

Reagan drew a bright autumn day of high clouds and bracing winds.

Mondale's sky was pewter-colored. Rain was in the air, and when the sun finally glimmered like a pale coin, it gave much relief to those who feared that Heaven was speaking as clearly as the Roman Catholic cardinal.

Mondale's crowd was more than twice the size of Reagan's. It was between 80,000 and 100,000, according to the Boston police. Democratic leaders, the governor, the two senators, the nearly monolithic congressional delegation, all seemed to feel personally challenged by the Reagan invasion. The word went out to the troops: Be there. And they were.

By 10 o'clock, the slope between the foot of Boston Common and the gold-domed State House was filled with eager, determined, aggrieved Democrats. While they waited for the standard-bearer, they tried to explain to visitors why Mondale was trailing in the Commonwealth polls, why Massachusetts might even send Ray Shamie, a one-time John Birch Society member, to the Senate to take the place of retiring Paul Tsongas.

Said a woman with big brown eyes and blond hair who had driven up from Cape Cod to take a stand: "I don't know what got into the Democrats this year. Maybe it's the hierarchy. A lot of people hate to go against them."

"I don't know," said another woman from Stoneham, where her Democratic friends and neighbors are going Republican for the first time. "They say Mondale is blah. I don't think so myself."

Reagan, standing on a platform heavy with faithless Democrats, quoted cavalierly from John F. Kennedy's elegant farewell message to the Massachusetts legislature. None of the Democratic orators so much as mentioned Jimmy Carter when they called the roll of Democrats whom Fritz Mondale should follow into the White House.

Reagan's admission-by-invitation rally was as well executed as a military maneuver. He arrived on the stroke of 11. His speech, even with a few asides to hecklers, finished at the appointed second. He was not for a moment subjected to the tedium of listening to other speakers. Shamie, whose surprising chances he supposedly came to boost, was not allowed to utter a word.

Reagan's music was provided by a high school band. Mondale's came from a black group, the Way of the Cross Gospel Choir, which sang "What do you do when something gets you down?" among other selections, and from Peter Yarrow and Mary Travis, who warmed the crowd to fever pitch with "This Land Is Your Land."

Mondale was an hour late. Before he got there, many had their say, including black City Council member Bruce Bolling, who spoke effusively of one of Mondale's heaviest burdens, Jesse L. Jackson. When Mondale took his seat on the platform, it was assumed he would want to hear such florid oratorical overweights as House Speaker Thomas P. (Tip) O'Neill Jr. and Sen. Edward M. Kennedy, both of whom delivered thunderous blasts of blarney that were joyously and boisterously received.

It didn't particularly matter that Mondale, when he finally got his chance, didn't even try to compete. He looked relaxed, even fresh. He was plainly in overdrive. Like other doomed candidates, he was taking comfort in a huge turnout, in the fun of partisan hyperbole, in hearing himself described, in Tsongas' words as "a very decent, real human being."

It may not have helped him elsewhere, but in Boston, it was enough to be a Democrat.