A special election in California will fill the seat vacated by Kevin McCarthy

Voters in the California district once represented by Kevin McCarthy, the ousted Speaker of the House of Representatives, will return to the polls on Tuesday to elect his interim replacement.

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

The two have already won a spot in November’s general election for a full term. But if Mr. Fong, who had a significant lead over Mr. Boudreaux earlier this month, can win 50 percent of the vote in Tuesday’s primary, he will head into November with the power of the incumbent at his side. If neither candidate wins 50 percent of the vote on Tuesday, they will instead compete again in a special runoff election on May 21 to temporarily fill the seat.

Democrats were shut out of the November election, with California’s “jungle” primary system meaning the top two winners advance to the general election regardless of party affiliation.

But Marisa Wood, a Democrat and teacher who finished third in the March 5 primary, trailing Mr. Boudreaux by about 4,000 votes, tried again on Tuesday. The ballot paper features a total of nine candidates.

However, Mr Fong remains the favorite on Tuesday and in the autumn. In the primaries, he defeated Mr. Boudreaux by nearly 20 percentage points. Mr. Fong has also dominated the field in fundraising, bringing in about three times the amount raised by Mr. Boudreaux. There is a super PAC affiliated with Mr. McCarthy 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 to the State Assembly since his election in 2016, could be a legal battle that has the potential to take him out of the general election.

In December, California’s secretary of state said he was ineligible to run because he had already filed reelection papers for his General Assembly district. Mr. Fong sued the foreign minister and won. The case is now being heard by an appeals court. The secretary of state has asked the court to make a decision by April 12, the deadline for certifying the votes from the primary elections.

Mr. Boudreaux, who served 37 years in the Tulare County sheriff’s office, has emphasized his conservative bona fides and made crime and illegal immigration the centerpiece of his campaign. Two of Mr. Boudreaux’s TV ads feature footage of him shaking hands with Mr. Trump, but Mr. Trump has not been as supportive in return — he endorsed Mr. Fong in February.

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