ST. CLOUD, MINN. -- The lovely image sketched by George Bush's managers -- a monolithic Republican Party united behind the vice president save for doctrinaire dissenters on the right -- was shattered by division and indecision at the Minnesota Republican Convention here.

It was no shock that Rep. Jack Kemp won newspaper straw votes of the delegates in a state whose GOP machinery has been dominated by evangelical conservatives for five years. The surprises last weekend were twofold: first, the substantial moderate minority backed not Bush but Sen. Robert Dole; second, Kemp was challenged on the right not by Dole but television evangelist Pat Robertson.

Some 1,000 delegates gathered at St. Cloud State University were neither united ideologically nor ironbound in their presidential choice. As an early selector of national convention delegates at caucuses next Feb. 23 (between New Hampshire and Super Tuesday), Minnesota in no way conforms to the cliche' that the nomination is the vice president's unless he somehow missteps.

On the contrary, this is one state where Kemp has been the clear favorite for early victory. His lieutenant, Rep. Vin Weber, returned from the 1984 Dallas convention to build support for Kemp among the Republican evangelicals who in the 1980s transformed the image of one of the most liberal Midwestern state parties.

But about six weeks ago, Dole began a Minnesota blitz, visiting the state six times prior to his convention appearance and aiming straight at Kemp's right-wing base. Two evangelical ministers are prominent in his organization. Moreover, his state chairman -- ex-state representative Cal Ludeman, who was last year's defeated nominee for governor -- had been counted on by Weber as a Kemp stalwart.

Ludeman's explanation to us reveals Dole's overriding argument here and elsewhere: ''electability.'' Ludeman told us he regards the senator as ''smart, maybe too smart; pragmatic, maybe too pragmatic.'' But he sees in Kemp some of the failings of his own defeated candidacy for governor: ''explaining too much, not able to say yes or no.'' Dole, says Ludeman, doesn't explain.

While protesting that ''I'll put my conservative record up against any candidate in the field,'' Dole declared to the convention: ''When all is said and done, the bottom line is who gets elected, and I think I'm electable.'' Half an hour later, Kemp told the same delegates, ''the key is not electability'' but ''policy and principle.'' Both got strong, prolonged applause.

Inexplicably, George Bush's well-organized, well-financed campaign has all but ignored Minnesota. His absence (he was in Warsaw, beginning a European trip) was compounded by little organizational effort here. A letter of greeting to the convention, written in the wooden prose common to the vice president's office, was read by his state chairman, former Minnesota House speaker Dave Jennings, as if it were an onerous chore. A promise to restore ''protection of the environment'' as a Republican issue was greeted by dead silence.

In contrast, although some supporters here felt Dole's new stump speech went on too long in autobiographical detail, it was highly effective. So was Kemp's more stirring oratory. Yet, some delegates were stirred even more by Robertson's telephoned speech, which transcended Kemp by calling for a rollback of communism in the Soviet Union itself and contradicted him by warning of the dangers of borrowing from Japanese and Germans. ''I'm for Jack Kemp,'' delegate Sig Lee, a drama professor at Bethany Lutheran College in Mankato, told us, ''but Roberston moves these people in a way Kemp does not.''

Marc Nuttle, Robertson's national campaign manager, had been working Minnesota for only a week but claimed one-quarter of the delegates. The straw vote figures of 13 percent and 19 percent showed Nuttle was not too far off. Minnesota, then, is the first state where Robertson shows strength beyond new people he has specifically brought into the party.

If Kemp's managers arrived in St. Cloud worrying about the Dole blitz, their worry on leaving was the enigmatic appeal of the evangelist from Virginia Beach.