A Michigan circuit court judge yesterday ruled illegal the efforts of backers of Marion G. (Pat) Robertson and Rep. Jack Kemp (R-N.Y.) to bar 1,200 party activists -- many of them supportive of Vice President Bush -- from participating in the selection of Michigan's 77 national convention delegates.

Although certain to be appealed, the ruling is a major victory for Bush, who has been locked in a bitter fight with the Robertson-Kemp alliance. Michigan has the potential to influence the outcome of Iowa and later contests because its Jan. 29-30 convention date makes it the first state to pick national convention delegates.

Speaking from the bench, Kent County Judge George Boucher directly criticized the Robertson-Kemp tactics in Michigan, declaring: "It is simply unfair to change the rules unilaterally in the middle of the game."

The judge's decision applies only to a dispute involving 1,200 men and women who won primary elections for state and local offices in 1986. But his wording clearly suggests that his interpretation of the law is broad enough to place in serious jeopardy a plan by Kemp-Robertson forces to enact a major change in party rules at a central committee meeting next weekend.

"It's going to strengthen our hand," said state Sen. John Engler, leader of Bush's Michigan drive. "I think it clearly means we can win," Lee Atwater, Bush's national campaign manager, said.

Laurie Parker, Michigan spokeswoman for the Robertson campaign, said she plans to recommend an appeal of Boucher's decision. In addition, she said the Kemp-Robertson forces will go ahead with the meeting next Saturday morning. Charles Black, Kemp's national campaign manager, agreed that an appeal will be made and the Saturday meeting will go ahead.

Black contended that federal courts, which will probably take ultimate jurisdiction over the case, have ruled in the opposite direction of Judge Boucher. Boucher stated that state party rules are superseded by state law and that "there seems to be no question here that state party rules are subordinate to national party rules."

The court fight grows out of a dispute over who is allowed to participate in a series of Jan. 14 county conventions in Michigan which, in turn, will select the delegates to the Jan. 29-30 state convention.

In August 1986, 9,000 "precinct delegates" were elected and they clearly are allowed to participate in the county conventions. In addition, state law, according to Bush and Judge Boucher, calls for the automatic participation of 1,200 men and women nominated to state and local offices. These 1,200 -- whom the Robertson-Kemp forces sought to bar from the county conventions -- are widely believed to lean in large numbers toward Bush.