ONLY hours after launching his presidential campaign today, Nebraska Sen. Bob Kerrey flies boldly into "enemy territory" in next-door Iowa, the home turf of his chief rival, Sen. Tom Harkin.A Kerrey-Harkin shootout between two of the Democrats' most promising presidential candidates now seems inevitable, and could be the pivotal contest in the 1992 nominating campaign. Political insiders say that Senator Kerrey and Senator Harkin - both Midwestern and liberal - will inevitably clash as they seek money and votes from the same group of Democratic activists who dominate the presidential primaries every four years. The contest could be explosive and colorful. Both men are effective politicians with strings of victories to their credit. Both are outspoken. Both lay claim to glittery Hollywood connections that could pump mega-bucks into their campaigns. Harkin is a take-no-prisoners campaigner. He enjoys political combat, rarely shies away from a fight, takes strong positions, and reminds many political observers of another scrappy campaigner, Harry Truman. Kerrey, a Medal of Honor winner, has wowed voters in traditionally Republican Nebraska, despite his oft-liberal voting record. Analysts say he runs persuasive campaigns of images which score well in elections. As James Rogers, a Nebraskan working toward his doctorate in political science at the University of Iowa, says of Kerrey's popularity: "Nebraskans say they don't like slick, good-looking politicians, but that's because we don't have many" like Kerrey. The Iowa presidential caucuses next Feb. 10 traditionally serve as the official start of voting in the presidential campaign, but 1992 may be different. Already, some Democratic candidates like Gov. L. Douglas Wilder of Virginia and former Sen. Paul Tsongas of Massachusetts appear to be scaling back their efforts here. With favorite-son Harkin in the race, why bother? But Kerrey, hailing from nearby Nebraska, apparently won't give up so easily. John Wright, a political scientist at the University of Iowa, says: "If I were Kerrey, I would certainly be coming in here and campaigning hard." Professor Wright explains that in past campaigns, Harkin has sometimes proven to be a "loose cannon," and if he stumbles, Kerrey will want to be in position to pick up the pieces. Wright adds: "If Harkin is seen the clear winner long before the race here starts, that is just an invitation for other candidates to come in here, for they don't have to win. All they have to do is run well against Harkin in his home state. That's how you get your campaign off the ground." In fact, there are already indications that Kerrey's camp is trying to set the stage to embarrass Harkin at home. Steve Jarding, Kerrey's press secretary, observes that in his US Senate race last year, Harkin got 90 percent of the Democratic vote in Iowa. "Harkin ought to get near 90 percent of the vote in the presidential caucuses," Mr. Jarding insists. Russell Ross, who has analyzed national political trends for over 40 years from his post at the University of Iowa, says Kerrey's strategy in Iowa may depend upon the reception he gets Tuesday at a rally here in Iowa City. But time is short. Ordinarily, presidential candidates spend months going from town to town in Iowa, attending church picnics, talking to Rotary clubs, sitting with townsfolk and farmers in cafes and talking about weather, crops, and the problems of the world. "It takes a long time to massage this state," says Wright. But the 1992 campaign is getting underway so late that time is running out. Even if Kerrey wants to mount a serious effort here, it may be impossible. Indeed, Hugh Winebrenner, an authority on the Iowa caucuses and professor at Drake University, doubts that Kerrey could get more than "minimal" support here, even with great effort. The caucus system is weighted toward favorite sons. Votes are cast in public, and for local party activists to oppose their own senator could be politically fatal. "You are talking about the Iowa senator [Harkin] who controls a significant portion of the Iowa Democratic Party, or at least his followers do, and to go on record against him is at your expense," Dr. Winebrenner says. Because of that, many analysts, such as Dr. Ross, expect Kerrey to eventually turn his attention elsewhere after a few forays into Iowa. One of the most important early primary states for Kerrey and Harkin could be South Dakota, which would provide a comparatively neutral battleground in the Midwest on Feb. 25, just two weeks after the Iowa caucuses. Kerrey obviously gives it great importance. After his Tuesday rally in Iowa, he will spend a day and a half courting Dakota voters. Meanwhile, New Hampshire is ready to reclaim its role as the preeminent early contest. Every Democratic candidate apparently will give it top priority.