A handful of Republican House members are among the largest contributors to a campaign to repeal California’s gas tax boost, one that could draw party voters to the polls in competitive congressional districts.

The seven lawmakers include House Speaker Paul D. Ryan of Wisconsin and second-term Rep. Mimi Walters of California. They have contributed or loaned more than $1 million of the $2.2 million received by a group called “Yes on 6, Repeal the Gas Tax,” according to second-quarter state election reports filed July 31.

The ballot initiative, known as Proposition 6, would reverse a 2017 state law that raised gasoline taxes by 12 cents per gallon and diesel taxes by 20 cents per gallon, with a vehicle registration fee, to pay for road repairs statewide and transit in the Los Angeles area.

With no competitive races at the top of the state ballot in an unfavorable midterm year for Republicans, the measure could boost turnout among otherwise disaffected GOP voters in a state vital to the party’s House majority. Larry Gerston, a political scientist at San Jose State University, said in a phone interview the ballot initiative allows Republicans to get out their voters to a greater degree than they would otherwise.

The difference in turnout could be a deciding factor for Walters’ and other close races, he added.