DETROIT -- Amid the indifference of voters and the annoyance of party leaders, conservative activist Clark Durant is nearing the end of a long campaign that goes to the heart of Republican listlessness today.

Durant is the underdog challenger in the Aug. 7 primary against the unofficial party choice, Rep. Bill Schuette, for the right to challenge formidable two-term Democratic Sen. Carl Levin. The few Republican voters interested enough to partake in the mid-summer balloting will decide a referendum on what kind of party they want.

Durant points to the similarities of Schuette's campaign with the failed Republican candidacies of 1986 and 1988, which led to Democratic control of the Senate. This sends a message the Republican hierarchy, here and in Washington, disdains. Durant is viewed by national party leaders as an unwanted diversion from their targeting of Levin.

But these leaders do not consider an upset out of the question. In a primary in which a mere 225,000 votes may mean victory and with all surveys showing well over 50 percent undecided, the unthinkable can happen. While the Schuette camp derides Durant for raising $1 million and not putting television commercials on the air until the final week, Durant aides talk about ''operating below the radar screen'' by identifying his voters and getting them to the polls.

Although only 41, Durant seems to have been around Michigan politics forever, making trouble. He walked Detroit precincts as a boy with his father, Richard Durant, a right-wing district leader locked in bitter combat with liberal Republican Gov. George Romney. Clark Durant was deeply involved in the tax platform fight at the 1984 Republican National Convention, assailed the American Bar Association as chairman of President Reagan's Legal Services Corp. and led Jack Kemp's campaign for president here.

Schuette, 36, began in politics as a delegate counter for the 1980 Bush presidential campaign and calls George Bush and James A. Baker III his ''role models.'' His principal accomplishment has been to reclaim a rural Michigan district from the Democrats in 1984, after which he settled for anonymity in the Republican congressional mainstream.

It would be a mistake to regard this as a continuation of Michigan's bloody presidential battling in 1988 or even as a strict ideological confrontation. Schuette and Durant divided up the old Bush and Kemp leaders (though Pat Robertson's activists are clearly in Durant's corner). Schuette takes positions scarcely less conservative than Durant's, scolding his old role model, President Bush, for breaking his word on taxes. He is intent on protecting himself from Durant, as shown by his vote last week to expel Rep. Barney Frank from Congress.

Schuette, in fact, is the 1990 model of the Republican moderate -- as different from the unabashed liberalism of Michigan's longtime GOP Gov. William Milliken as the George Bush of 1990 is different from the George Bush of 1970. Schuette sounds like today's Bush; Durant sounds like Kemp. While Schuette takes a carefully hedged pro-life position, Durant declares that ''I think the cause for unborn children is pre-eminent.'' He calls for ''empowering people,'' advocates new tax cuts and brands Schuette as part of the congressional crowd that led to the savings and loan disaster.

Schuette ignores both philosophy and Durant, concentrating on Levin. He treats Durant as an unperson, refusing to debate him. In an interview, he sounded like his commercials: ''Who's going to raise taxes? Not Bill Schuette. Carl Levin has in the past. He's going to in the future.''

To Durant, that is the wrong formula. ''We're going to win,'' he tells audiences, ''not by calling Carl Levin a 'radic-lib,' not by beating up on him for being opposed to the death penalty, not by calling him Mike Dukakis without a helmet.'' He calls for a philosophical break into the blue-collar vote that could end Republican defeats in Michigan.

Wendy Lamb, his campaign manager, is a 1988 Bush activist who backs Durant because ''I'm tired of following the same losing candidates over and over again.'' Schuette looked to her like another bright upper-class Republican who could never win over the blue collars.

Lamb ventured to Kennebunkport to urge the president's neutrality, and Bush stayed out. But there is no doubt where he stands. Commerce Secretary Robert Mosbacher, Agriculture Secretary Clayton Yuetter and Veterans Secretary Edward Derwinski have campaigned for Schuette, as has George W. Bush, the president's son.

Housing Secretary Kemp has not opposed his Cabinet colleagues. Rep. Vin Weber of Minnesota, who battled shoulder-to-shoulder with Durant on the 1984 platform and in the Kemp campaign, came here in support of Schuette.