Democrats have not won a governor’s race since 1998, and Ms. Abrams would not only be the first black governor here — she would also be the first black female governor in the country. She ran in her own primary as a down-the-line liberal, criticizing other Democrats who have sought to win by tailoring their message to appeal to the state’s rural conservatives.

But even if Republicans do maintain an advantage in Georgia, the nomination of Mr. Kemp in a year when suburban woman are flocking to Democrats could, at a minimum, force G.O.P. groups to spend money in a state they have safely counted in their column in recent years.

What is clear is that Mr. Kemp’s ads, in which he boasted about being “a politically incorrect conservative,” appealed far more to grass-roots conservatives in this Trumpian moment than Mr. Cagle’s decades-long experience in state government.

The commercials, as with most everything Trump-related, prompted eye rolling from establishment-aligned Republicans in private. And one of those Republicans snickering was Mr. Cagle himself.

Mr. Cagle, the lieutenant governor since 2007, spent much of this year’s race as the Republican front-runner. Yet well before Mr. Trump intervened in the campaign on Mr. Kemp’s behalf last week, Mr. Cagle encountered difficulty when he was recorded talking in blunt terms to a former opponent — recordings that were leaked out in extended and excruciating fashion.

Mr. Cagle was heard on tape saying the Republican primary had become a test of “who had the biggest gun, who had the biggest truck, and who could be the craziest.” And he conceded that he had supported what he believed to be bad public policy in an effort to thwart an opponent’s fund-raising efforts and boost his own.