As quietly as snow falls on Iowa cornfields, change is again settling over the way Americans nominate their presidents. The trend toward primaries has been reversed. At least nine states that held primaries in 1980 are weighing a switch to caucuses for 1984.
The movement now is in Iowa's direction, toward ''caucuses,'' which Iowa will be the first to hold, in February 1984. Caucuses are local party gatherings - 2, 800 in Iowa alone, held in farmhouses and firehouses - that begin the process of determining a candidate's strength on a state delegation to the national party nominating conventions.
In a precinct caucus like Iowa's, party members split into groups to support their candidate. Each candidate's proportionate strength at the local level is eventually reflected in that state's delegation to the national convention.
The trend is away from the New Hampshire model - the primary, where statewide party ballots determine the candidates' convention strengths.
What this means is a turning back from the primary trend, and likely, from the prospect of a regional or national primary system often mentioned as a basic election reform.
The emphasis now is on party building at the state and local level, which caucuses enhance. And the shift to caucuses is another side of the determined comeback of the national Democratic and Republican Parties, whose strengths have ebbed in recent years.
By mid-April state parties must file either a caucus or primary plan for selecting delegates with national party headquarters. Louise Lindblom, delegate selection chief for the Democratic National Committee, says at least six states that held primaries in 1980 will likley switch to caucuses in 1984 - Arkansas, Kansas, Kentucky, Montana, Nevada, and Tennessee. Also California, Ohio, and Wisconsin are considering caucuses for 1984. One state, North Dakota, might switch from a caucus to a primary.
This would mean 31 caucuses and 25 primaries in early '84 for the 50 states plus the District of Columbia and five other party voting units, such as Puerto Rico and Guam. In 1980 there were 31 primaries and 25 caucuses for the 56 delegate events. (In '84 there will again be at least three additional ''beauty contest'' events, which will not decide convention delegates).
Officials of both parties here in Iowa speak enthusiastically of their caucuses - a view shared by the national politicians who hope to exploit it.
On the Republican side, Sens. Howard -Baker of Tennessee and Robert Dole of Kansas have signaled their availability to speak at Iowa GOP events this spring - no doubt to be ready to run should President Reagan say no to one more term. Rep. Jack Kemp of New York says no for now, but try him later, according to Iowa Republican Party chairman Rolf Craft.
Vice-President George Bush's 1980 Iowa campaign operation was kept intact after his presidential bid, Mr. Craft says, and is ready to roll again. Should Reagan decline a second term, the longer he waits to make such an announcement, the greater Bush's Iowa advantage over the others in campaign start-up time.
Among Democrats, five already have operations under way in Iowa, with former Vice-President Walter Mondale and Sen. Gary Hart of Colorado the best prepared.
''The sheer numbers of people who come out of the woodwork to participate in caucuses can blow your mind,'' says Craft of a hotly contested nomination like 1980. ''In my own town of Decorah, with a population of 8,000, 35 or 40 people ordinarily might show up,'' Crafts says. ''All of a sudden we had over 1,000. If Reagan does run, we'll have a very quiet night. But if he doesn't, it'll be very lively. There's no chance Reagan could be upset in Iowa if he wants the nomination again,'' Craft says. ''But if he doesn't run, Bush has a leg up. His organization is still in place - laying dormant. Bush becomes an overwhelming favorite in case of a late announcement.''
Women's issues could prove the toughest for the GOP in Iowa, says Craft. And Mary Iseminger, a GOP activist from Hudson, Iowa, agrees.
''The fight in Iowa will be between the ultraconservatives and the moderates, '' says Mrs. Iseminger of the internal Iowa party struggle ahead.
''If they make the abortion fight public here, the party will lose moderate Republicans and independents, and Republicans will go down in Iowa. The women's vote is distinctively not going for the Republicans. (Reagan) wants to go back to a mythical time that women don't want to go back to. His appointments have been anti-women.''
The Iowa GOP caucuses seem headed for sharp disputes, particularly if Reagan abandons the field, which could turn out single--issue conservatives in full force.
''You just cringe when you go to a meeting and hear them talking about teaching creationism in the schools. We always vote the diehards down, but there are a lot of them and they know how to use the caucus system,'' say Mrs. Iseminger.
Some 100,000 Iowans joined in the Democratic Party's 1980 caucuses, says Karen Kapler, executive director of the state Democratic Party. That number could be equaled or bettered in 1984, particularly if again there are contested nominations in both parties.
''Candidates prefer primaries because of the publicity they get,'' says Carol Casey, formerly a Library of Congress expert on party nominations. ''Caucus states take more time to organize, more bodies. But caususes are cheaper because you don't need as much television.
''Caucuses are much better for party building,'' she says. ''Voters have a chance to meet and talk about issues, sign up for party activities. They can make suggestions for the party platform. They can get to know each other better than if they step into a booth and pull a lever (in primary balloting).
''One argument against caucuses is that they are elitist - a much smaller group attends. But studies show the type of people - in all their demographic varieties - who attend caucuses are the same type that vote in primaries.''