IOWA, where the presidential campaign usually gets its start, is feeling left out of the 1992 race.Four years ago, presidential candidates were swarming all over Iowa like grasshoppers. Remember Babbitt? Gephardt? Gore? Hart? Jackson? Schroeder? Simon? Dukakis? Biden? Bumpers? And those were just the Democrats. Republicans were nearly as plentiful. Four years ago, TV crews and newspaper reporters were tromping through cornfields and hog farms looking for farmers to interview. They were quizzing experts about the idiosyncrasies of Iowa politics, and running polls to see who was gaining ground. This year, the news media are mostly back in places like Washington and New York, or up in New Hampshire. By this time in the previous presidential race, the Democratic candidates had spent a total of 385 days in Iowa. United States Rep. Richard Gephardt of Missouri held the record at the time with 87 days. This year, all the candidates together have campaigned fewer than 50 days. The leader is former Sen. Paul Tsongas of Massachusetts with 26 days. What happened? Two things, mostly. In the 1988 election, Iowa lost some of its reputation as a kingmaker. George Bush did poorly in Iowa, and Sen. Robert Dole did well, but eventually Mr. Bush ran away with the nomination in spite of his Iowa loss. Much the same thing happened on the Democratic side. Winning in Iowa suddenly didn't seem so important. The other change was Tom Harkin's entry into the Iowa presidential caucuses. Senator Harkin, who comes from Iowa, is expected to get most of the votes as a favorite son. So several other candidates, figuring they can do better in New Hampshire, are avoiding this state. Media hoopla in past years has helped to increase turnout at the Democratic caucuses to around 100,000. But the dearth of candidates and the decline of media coverage will probably reverse that trend, which began with the unexpected victory of an obscure former Georgia governor, Jimmy Carter. Hugh Winebrenner, an expert on the Iowa caucus process, expects attendance to drop below 50,000 in 1992.