IN GRAND RAPIDS last week, George Bush won a decision in the second of two lawsuits he has brought challenging the Michigan party rules set by supporters of Pat Robertson and Jack Kemp. In Detroit yesterday, he persuaded a federal judge to dismiss a case challenging the state election law. Unless these decisions are overturned by appeals courts today or tomorrow, the Bush campaign will have a good chance at tomorrow night's 83 county conventions of electing enough delegates to the state convention to put its candidate in the No. 1 position, ahead of Mr. Robertson and Mr. Kemp.

Since the state convention in Michigan is only a little more than two weeks away (Jan. 29), that would give Mr. Bush the first victory of the presidential year in a contest in which he seemed for a while likely to run third. The Bush forces elected more precinct delegates in the August 1986 primary -- the last crack ordinary voters got at this contest -- than did any of his rivals. But a Robertson-Kemp coalition, to which the Robertson forces contributed most of the numbers and the Kemp forces most of the leadership, captured control of the party apparatus in a February 1987 state convention.

It was two of the Robertson-Kemp state central committee's acts that the court in Grand Rapids overturned: a move to block the 1,200 mostly pro-Bush local and party officials from voting at county conventions and a move to hold most of the districts from which state convention delegates were elected to the same geographical boundaries used last February. The Robertson-Kemp coalition argued that state parties are a law unto themselves, citing Supreme Court cases that say national convention delegates aren't bound by state law. But the courts have said that state parties are bound by state law. This seems pretty sensible to us.

So in frigid Michigan, the phone lines are blazing. The Bush and Robertson campaigns urge their precinct delegates to attend their conventions. Both are fighting for the allegiance of Kemp delegates. Could the first result of 1988, one which could set the tone for the whole Republican race, be determined by the decisions of two Grand Rapids judges, by the agonizing of a few Kemp delegates or by whose cars will start on a cold Michigan night and get them to the county convention on time?