For the third time this year, voters in California’s most conservative congressional district will choose whom they want to replace Kevin McCarthy, the ousted speaker of the House — and this time the winner will head straight to Washington.

In a special runoff election on Tuesday, voters will determine who will finish Mr. McCarthy’s term for the remainder of the congressional session. The choice is between two Republicans: Vince Fong, a state assemblyman and former aide to Mr. McCarthy, and Mike Boudreaux, the sheriff of Tulare County. The two men qualified for Tuesday’s runoff after a special election in March in which they were the two top finishers, but neither candidate took 50 percent of the vote.

Mr. Fong entered Tuesday as the favorite. He has benefited not just from Mr. McCarthy’s estimable sway in the Bakersfield region, but also from an endorsement from former President Donald J. Trump.

The winner of the special election will have the benefit of incumbency in the November election for a full term representing the southern end of the Central Valley in the next congressional session. The same candidates, Mr. Fong and Mr. Boudreaux, are facing off in that contest after emerging as the top two finishers in a separate March primary contest. Mr. Boudreaux has indicated that he could decide to withdraw from the November race if he does not win Tuesday, according to KFSN-TV, a Fresno-based ABC outlet.

Both candidates have called for tighter border controls, abortion restrictions, more California oil production and more water for farmers. Mr. Boudreaux, who serves as the president of the California State Sheriffs’ Association, has emphasized his experience on public safety. Mr. Fong, who is the vice chairman of the California State Assembly budget committee, has highlighted economic issues, saying he would oppose new taxes and reduce regulations on businesses.

The two March votes locked Democrats out of the seat, as the state’s “jungle” primary system allows the two top finishers to advance to the general election regardless of party affiliation.

Mr. Fong secured about 42 percent of the vote in each of the March elections. Mr. Boudreaux won 24 percent of the vote in the primary, and made an incremental gain of two percentage points in the first contest to finish Mr. McCarthy’s term. Mr. Fong is also ahead in fund-raising, having pulled in about $1.5 million, more than three times the roughly $425,000 raised by Mr. Boudreaux.

Mr. Fong fought off an earlier threat: an initial determination in April by the California secretary of state that he was ineligible to run for Congress because he had already submitted re-election paperwork for his Assembly seat. Mr. Fong sued the secretary of state and state courts determined he could remain on the congressional ballot.