LANSING, MICH. -- The powerful alliance between Marion G. (Pat) Robertson and Rep. Jack Kemp (R-N.Y.) that dominated Michigan Republican politics for the past 14 months began to crumble today as two of three Kemp cochairmen resigned to form a coalition of convenience with the campaign of Vice President Bush.

The two dissident Kemp backers worked out a deal with local Bush supporters that, if successful, would give Bush a plurality of the state's 77 national convention delegates and allow Kemp to take second place, with the rest going to Robertson.

While the deal could produce a significant number of delegates for Kemp, who received only about 10 percent in a key August 1986 vote here, it was disavowed by the candidate. State Sen. Dick Posthumus and Larrain Thomas, the two defectors, both affirmed their continuing support for Kemp, but their resignations as Kemp state cochairmen were promptly accepted.

The battle over Michigan's delegates is significant because its Jan. 29-30 convention makes it first in the nation to select GOP delegates.

In addition, Michigan has become a key testing ground for the Robertson campaign. For a time, it appeared that Robertson's ability to mobilize his Christian supporters had been so effective that he was going to win a majority of Michigan's delegates under the Byzantine system developed by the state's GOP. Now, however, the advantage appears to have shifted to Bush.

"This clearly gives us some life we haven't had in a while," Lee Atwater, Bush's national campaign manager, said. He cautioned, however, that he has been involved with the Michigan delegate-selection process for three years, and "it's like the trick birthday candle, every time you think something is done, it comes right back up again."

The announcement by Posthumus and Thomas, who said they will be joined by 11 of 18 Kemp district chairmen in Michigan, is the third, and by far the most important, recent development favoring the Bush drive here.

Last week, 11 Robertson supporters -- none major players -- announced that they were joining Bush because they were disturbed by Robertson's alleged use of Christian Broadcast Network money for his campaign. That was followed by a circuit court decision barring the Robertson-Kemp alliance from adopting a plan to keep 1,200 largely pro-Bush GOP activists from participating in the county and state convention.

"Obviously, we are {hurt}," Charlie Black, Kemp's national campaign manager, said.

Not only did today's events suggest that Kemp cannot control his own supporters, but two of his Michigan supporters, Dick Minard and Clark Durant, were forced to come here to publicly promise local Robertson supporters that Kemp himself "has no intention of giving up on his commitment to the conservative {Robertson-Kemp} coalition."

The weakness of the Kemp campaign, both here and nationally, appeared to be a major factor in the decision of the dissident Kemp supporters to form an alliance with the Bush campaign. The Bush-vs.-Robertson-Kemp fight here has severely strained the Michigan Republican Party, and sources involved in the negotiations indicated that Posthumus and Thomas, both party regulars, had become unwilling to allow the wounds to get worse for a Kemp presidential campaign that has yet to take off.

Kemp, according to Black, has borrowed nearly $2 million against expected federal matching money, has issued another $300,000 in letters of credit and is carrying a debt to vendors of about $500,000. Black claimed that the campaign will receive enough additional matching money to add $1 million to the coffers as of Jan. 1. But in the meantime, Kemp has failed to gain in polls, and one recent one in New Hampshire, a vital state for him, shows him dropping to fourth place, behind Bush, Senate Minority Leader Robert J. Dole (Kan.), and now, former Delaware governor Pierre S. (Pete) du Pont IV.

Asked whether the defections represent a major setback, Robertson's Michigan spokeswoman, Lori Packer, said, "I can't say yet. I wish I could say no, but I have to take a look."

At a joint press conference, Posthumus and State Senate President John Engler, head of the Michigan Bush campaign, said that if all goes according to plan, Bush will receive a plurality of the 77 Michigan delegates and Kemp will receive three or four fewer, with the remainder going to Robertson.

The first test of the bizzare new battle between Bush-Kemp and Robertson-Kemp will be at a Michigan GOP state central committee meeting Saturday. Robertson and his remaining Kemp backers intend to seek a major rules change opposed by pro-Bush forces. Before today's announcement, Bush controlled only 32 votes on the 100-member central committee, while 58 were firmly with the Kemp-Robertson alliance and about 10 were not formally aligned. With the new alliance, the Bush forces hope to stop the rules change.