After the high drama of last winter and the despondency of last summer, Iowa's Democrats are going into the last three weeks of the long 1980 campaign in a mood of down-home determination.

The atmosphere at the Jefferson-Jackson Day dinner here Saturday night was as different from last June's state convention as that convention was from the night of the Iowa precinct caucuses in January -- when the seemingly endless political year began.

In the intervening nine months Iowa Democrats have ridden an emotional roller-coaster, from a belief that the record turnouts at the caucuses spelled a possible boom year to a feeling at the June convention that "we could lose it all."

Dagmar Vidal, the just-retired national committeewoman who offered that gloomy forecast in June, said yesterday, "It's looking better." She spoke as the Democrats once again filled the Veterans Memorial Auditorium, this time to hear a pep talk by Vice President Mondale.

But, as it turned out, Vidal was not talking about the presidential contest, which had been the focus of January's excitement and the source of much of the gloom in June.

President Carter has narrowed Ronald Reagan's lead to 12 points in the latest Iowa Poll, published in today's editions of the Des Moines Register and Tribune.

Yet the real tonic for the spirits of the Democrats was a report that the same poll would show that Sen. John C. Culver, who was running nine points behind at the time of the June convention, is now five points ahead of his Republican challenger, Rep. Charles Grassley.

"When people talk about how the race is going," said Maria Menne, a staff member of the Democratic state committee, "they mean the Culver-Grassley race -- not the presidential race."

The focusing of energy on the Iowa contest is one of the big changes that strikes a visitor. No one minded that Mondale made his speech before dinner was served and then flew on to Wisconsin. The speech they had come for -- and the one they really wanted to cheer -- was the stemwinder that Culver delivered after dessert.

The presidential race has been an expensive, exhausting journey for the Iowans -- and it is one they will not be sorry to see ended. When Sen. Edward M. Kennedy chose the Iowa caucuses as the site of his first major challenge to Carter's renomination, the state's Democrats became pawns in a war few of them willingly chose.

Old friends found themselves on opposite sides, as party factions and such important allies as the United Auto Workers and the Iowa Education Association threw all their resources into the struggle to turn out voters for Kennedy or Carter at the precinct caucuses.

Carter won by a 3-to-2 ratio, but the infighting continued right through the week of the Democratic National Convention in August -- a trying week for the Iowa delegates, whose internal quarrels were filmed by the television networks and reported in detail by hometown newspapers.

"I would not want to go through that kind of pressure again," said one of the teacher-delegates for Carter.

But at the end of that week in New York the Iowans had what state chairman Ed Campbell called, "almost a political witnessing ceremony, where we pledged to each other that this would not be the end of our working together."

The Culver campaign has been the vehicle for that process of reunification and rededication. The senator is engaged in a classic liberal vs. conservative struggle with Grassleyy, and heads nodded in agreement when Culver told Saturday night's dinner that the election is "crucial for the political future of our state."

Two years ago, conservative Roger Jepsen upset the state's other liberal Democratic senator, Dick Clark, with the help of anti-abortion and fundamentalist religious groups, who now are mobilizing just as strongly against Culver.

"The turnout in 1978 was only 38 percent," Campbell said, "and we cannot be forgiven if John loses because we failed to get out the vote again."

The shift of emphasis to the Culver race has been made easier by the contrast between his campaign style and Carter's. Culver has been defending his record in unapologetic terms, drawing cheers Saturday night with impassioned support of foreign aid and arms control. Carter's personal attacks on Reagan, meanwhile, have been viewed with dismay even by his early supporters here.

"I called Tim Kraft just before he resigned," said Floyd Gillotti, "and told him to clean up those ads. People expect better of an incumbent."

Vidal, another early backer of the president, said: "I've told them he can win this election, if he will just stop being mean."

The rally-around-Culver spirit has worked to the advantage of other Democrats and even may help Carter in his uphill fight to carry Iowa. Lyn Cutler, a Blackhawk County commissioner and Democratic congressional candidate, for example, had been afraid that her work for Carter in the caucuses would hurt her standing with the pro-Kennedy UAW, a major force in her district.

"But they have been just great," she said, and she is given a chance of winning in a historically Republican district.

On the other side, Tom Miller, the state attorney general and cochairman of the Kennedy effort last winter, was out at the airport Saturday to greet Mondale. "He's family in this state," Miller said of the Minnesota-born vice president, who has visited Iowa so often this past year that a Culver aide remarked "he might as well be the Senate candidate."

After all the tribulations of the last nine months, it is as a family that Iowa's Democrats are winding up this campaign. And win or lose, they feel better being back together.