Mr. Kemp spoke briefly, sticking to his vows to bring an outsider’s perspective, prioritize small-business owners over special interests and embrace a tougher stance on illegal immigration. He also declared he would be the Republicans’ best shot at defeating the Democratic candidate, Stacey Abrams, a former state lawmaker who is trying to become the nation’s first black female governor.

“Georgians are sick and tired of these politically correct liberals like Stacey Abrams who are offended and outraged by our faith, our guns and our big trucks,” Mr. Kemp said.

The rally was one of the first in which Mr. Kemp appeared as the front-runner in Tuesday’s runoff. Mr. Cagle has stumbled since leading the five-candidate primary by double digits two months ago. In a secretly recorded conversation that was made public this summer, he said that the primary had become about “who could be the craziest.” Another recording, released in late spring, revealed that he had supported a bad bill to undermine a political rival.

Some political observers wonder whether Mr. Kemp will adopt a more centrist tone — in an attempt to win moderate voters — should he defeat Mr. Cagle. But Charles Bullock III, a professor of political science at the University of Georgia, said Georgia’s demographics make it possible for Mr. Kemp to win the general election without softening his campaign’s strong rhetoric, which Democrats have criticized as divisive.

If Mr. Kemp wins the runoff, he will talk about attracting suburban voters, said Kerwin Swint, a professor of political science at Kennesaw State University. “But the biggest part of his campaign,” Mr. Swint said, “will be cranking out the Republican base — Trump voters. And being in the Trump tent, so to speak, will allow him to really crank that up.”