Voters in the California district once represented by Kevin McCarthy, the ousted speaker of the House, will head back to the polls Tuesday to choose his interim replacement.

The front-runners are Vince Fong, a state lawmaker and former aide to Mr. McCarthy who has won the backing of former President Donald J. Trump, and Mike Boudreaux, the longtime sheriff of Tulare County.

The two have already nailed down spots in the November general election for a full term. But if Mr. Fong, who had a hefty lead over Mr. Boudreaux earlier this month, can clinch 50 percent of the votes in the primary Tuesday, he will head into November with the power of incumbency on his side. If neither candidate wins 50 percent of the vote Tuesday, they will instead compete again in a special runoff election on May 21 to fill the seat temporarily.

Democrats were locked out of the November election, with California’s “jungle” primary system meaning the two top finishers go on to the general election regardless of their party affiliation.

But Marisa Wood, a Democrat and a teacher who came in third in the primary on March 5, lagging behind Mr. Boudreaux by around 4,000 votes, was trying again on Tuesday. The ballot lists a total of nine candidates.

Mr. Fong remains the favorite, though, on Tuesday and in the fall. In the primary, he bested Mr. Boudreaux by nearly 20 percentage points. Mr. Fong has also dominated the field in fund-raising, pulling in around triple the amount Mr. Boudreaux has raised. A super PAC affiliated with Mr. McCarthy has spent more than $650,000 to lift Mr. Fong.

The biggest threat to Mr. Fong, a longtime ally of Mr. McCarthy who has represented Kern County in the State Assembly since his election in 2016, may be a legal battle that has the potential to knock him off general-election ballots.

In December, California’s secretary of state said he was ineligible to run because he had already filed re-election paperwork for his General Assembly district. Mr. Fong sued the secretary of state and won. The case is now being considered by an appeals court. The secretary of state has asked the court to decide by April 12, the deadline to certify votes from the primary.

Mr. Boudreaux, who has served in the Tulare County sheriff’s office for 37 years, has highlighted his conservative bona fides, making crime and illegal immigration centerpieces of his campaign. Two of Mr. Boudreaux’s TV ads include footage of him shaking hands with Mr. Trump, but Mr. Trump hasn’t been quite so supportive in return — he endorsed Mr. Fong in February.

Ms. Wood, the top-finishing Democrat on March 5, has never held elected office before. Her campaign has raised less than $70,000.