GRAND RAPIDS, MICH. -- Steamrollering Pat Robertson's forces so that George Bush could have a plurality of the first Republican National Convention delegates was no surprise at last weekend's state GOP convention, but the anguish of Rep. Jack Kemp's supporters was.

Getting 32 delegates to Bush's 37 (while Robertson was stomped with only 8) hardly cheered Kemp's backers. From leaders to rank-and-file, most were furious that he had split the pie with the hated Bush establishmentarians they have battled for three years in his behalf.

To exploit this disillusionment, Robertson dramatically arrived to address the right-wing rump that bolted the regular convention. In denouncing ''back-room deals'' and ''sellout of principle,'' the religious broadcaster wants to use what happened here to win support beyond his churches. Although Robertson is likely to fail, Kemp made a grievous mistake here just as he was emerging as national leader of the Republican right.

Actually, Kemp was victimized by events: refusal by stubborn Michigan regulars to permit a Robertson victory, unfavorable court decisions and defection to a new Bush-Kemp coalition in December by a splinter of Kemp delegates in defiance of their candidate's wishes. Kemp followed his national advisers and played the hand dealt him, so he could name his delegates going to New Orleans.

That advice came from campaign chairman Ed Rollins and campaign manager Charley Black, nationally experienced pros. But Kemp's followers here are not professional politicians; they are movement conservatives. Disagreeing with Black's dictum that ''a delegate is a delegate,'' they are a different breed from Bush backers, who want a winner above ideology.

The conservatives fought for a candidate still in single digits in national polls because they believe Jack Kemp enunciates their ideals better than anybody else. Veterans compared their sense of betrayal with Ronald Reagan's 1976 selection of liberal Republican Richard Schweiker as his running mate.

Paul Gadola, a Grand Rapids lawyer who long has supported the conservative cause at personal cost, was a Reagan supporter in 1976 against incumbent President Gerald Ford in his home state. Gadola's decision to back Kemp in 1985 froze his progress toward a federal judgeship. Last weekend, he declined a seat on the Kemp delegation and joined Robertson's rump convention (while still supporting Kemp).

So did Kemp's most militant stalwarts here, including longtime supporter Clark Durant. Furthermore, Kemp strategists had no cause for pleasure over how many delegates stayed on the ground floor of the Grand Center with the regulars rather than move downstairs with the rump.

We found remorse upstairs among such Kemp delegates as Mike Goschka of St. Charles ("My head is up here but my heart's down there"). An hourly employee at Dow Chemical, he exemplifies populist revolt against the Republican establishment. ''This campaign has sold out to George Bush,'' Goschka told us, ''and Jack Kemp let them do it.''

Four Kemp county chairmen and one district chairman defected to Robertson on the spot. Two suburban Detroit stalwarts, Michael Legg and Elaine Donnelly, stripped off their Kemp buttons after being unseated by the regulars.

''I can't believe this deal for 30 pieces of silver,'' said nationally renowned activist Donnelly. ''There's no deal,'' Rollins told Donnelly in a heated exchange. Rollins promised Legg a meeting with Kemp, at least delaying a Robertson endorsement.

In truth, purging Legg's 2nd Congressional District delegation violated instructions from national deputy campaign manager Rich Bond. Bush's Michigan leaders admit they were retaliating against Legg's collaboration with Robertson forces. ''He deserved it,'' Peter Secchia, the fiercely pro-Bush Republican national committeeman, told us.

That fit a convention strategy intended to show the country how to handle Robertson interlopers. A written agreement giving Sen. Robert Dole one delegate was breached so it could be said Bush's principal rival was shut out in Michigan. While the Robertson rump debated school questions, the regular convention supinely accepted the reelection of the controversial Secchia by voice vote and adopted new party rules to impede future insurgency here.

Robertson will cite such excesses to attack ''party bosses,'' challenging Kemp's credibility on the right. Kemp's managers erred by running a tactical campaign as prosaic as the Bush and Dole campaigns, when in fact he can prosper only as the authentic leader of the conservative movement.