Mr. Cagle, who has been lieutenant governor for nearly 12 years, finished first in a crowded Republican primary last month but now faces a worrisome challenge from Georgia’s secretary of state, Brian Kemp. The winner advances to a November general election against Stacey Abrams, a Democratic state representative who is drawing national attention and money as the first African-American woman to win a major party’s nomination for governor.

Image Terry E. Hobbs, the lobbyist who owned the apartment. He had used the unit as a “hospitality room” for legislators.

The contest is seen as an important gauge of how quickly states like Georgia, with growing minority and immigrant voting strength, could shift from red toward blue. It may also provide clues about the cohesion of President Trump’s coalition in a Southern state that is both urban and rural, and about whether Democrats can win in the region by tacking left to energize their base.

The Cagle campaign suffered an embarrassment last week when one of the lieutenant governor’s vanquished opponents released secretly recorded audio of their private meeting to discuss a possible endorsement. In the recording, Mr. Cagle explains a recent dispute over tax credits for private school scholarships in brazen political terms, acknowledging that he had supported legislation he considered “bad public policy” purely to undercut the fund-raising of a former rival.

Mr. Cagle’s leads in polling and fund-raising have made him the candidate to beat in this race. At 52, he is running as a steady conservative with long experience in state government — he first was elected to the Senate in 1994 at age 28 — and as a small-business owner, entrepreneur and investor.

After leaving Georgia Southern University when an injury ended his football career, he returned to Hall County, an hour’s drive northeast of Atlanta, where his family has lived for seven generations. He managed and then bought a tuxedo rental shop, founded a small bank (quadrupling his initial $50,000 stake when it was acquired by a larger bank five years later) and invested in rental property and other real estate.

Mr. Cagle aborted his first race for governor in 2009 after announcing he had a degenerative spinal condition that required surgery. His withdrawal left an opening for another Hall County resident, Nathan Deal, to run and become the state’s current two-term governor.

Mr. Kemp, 54, who has served two terms as secretary of state, separated himself from the Republican primary pack with lighthearted ads that emphasized his political incorrectness. He has some momentum but is vulnerable to Mr. Cagle’s attacks on competence. In 2015, Mr. Kemp, whose office oversees state elections, took responsibility for a data breach that resulted in the release of the Social Security numbers and birth dates of six million Georgia voters.