This is the time of the campaign when interpreting primary results gets a little tricky. The public is told the races are about over, and people go into the voting booths and do all kinds of unpredictable things.

Take Michigan. Grand Bush's grand upset of Ronald Reagan there yesterday did little to loosen Reagan's grip on the Republican nomination. But it must be a little unsettling for Reagan's staff, which has just begun to get serious about the fall campaign.

Reagan's bold bid for Democratic votes, the key to an industrial-state strategy that he hopes will carry him to victory in the fall, ran aground in Michigan, as Bush walloped the former California governor in the state that symbolizes blue-collar America. In fact, Bush beat Reagan among all categories of voters.

Voter turnout in Michigan was extremely light, which makes it more difficult to extrapolate from the results there. And Bush outspent and overworked Reagan in the state and enjoyed the support of popular Gov. William Milliken.

The defeat takes some of the glamor away from the Reagan candidacy, which for the last month or so has enjoyed a kind of larger-than-life status. Reagan will have to start proving again that he has the broad appeal necessary to win in November.