When California Gov. Jerry Brown signed into law last spring a 12-cent gas tax increase and a new transportation improvement fee, Democrats hailed it as a way to fix the state's ailing roads and improve public transit.

But a little more than a year later – and with a gallon of gasoline costing about 80 cents more than the national average – some Democrats in key congressional races are siding with Proposition 6, a GOP-led ballot initiative to do away with the gas tax hike.

"I support Proposition 6, which repeals the gas tax,” Josh Harder, a Democrat and venture capitalist challenging incumbent GOP Rep. Jeff Denham for a seat representing part of the state’s Central Valley, told the Los Angeles Times. “We all agree that we need to fix our roads and bridges, especially here in the valley, but it should be through a thoughtful, cost-effective national plan, not through another tax we can’t afford like a gas tax that hurts working families.”

Harder is one of at least four Democrats running for Congress who are bucking their party and supporting Proposition 6. Many of the candidates are running in Republican strongholds like Orange County and the Central Valley.

The gas-tax increase -- along with a vehicle fee -- was approved by the state’s Democrat-controlled legislature and signed into law in April 2017 by Brown to generate roughly $5.2 billion annually to improve state infrastructure.

Supporters say the money is badly needed to fix the state’s crumbling roads, bridges and other infrastructure and that, if it passes, November’s ballot measure would jeopardize not only road improvements, but public safety.

"This ballot measure would jeopardize public safety by eliminating funding for vital highway, road and bridge safety improvements across California. It's dangerous and must be defeated," Doug Villars, president of the California Association of Highway Patrolmen, part of a coalition of labor unions, construction contractors and local governments, told Fox News earlier this year.

Critics of the tax, however, argue that the state’s soaring gas prices – one notorious gas station in Los Angeles is selling a gallon of regular for close to $5 – paired with gross economic mismanagement have put an undue burden on Californians and led to little change in the quality of the state’s infrastructure.

“This gas tax was not necessary and it affects anyone who drives,” Matt Fleming, a spokesperson for the California GOP, told Fox News. “Which in California is basically anyone who lives outside of San Francisco.”

A USC Dornsife/Los Angeles Times statewide poll conducted back in May found that 51 percent of Californians supported repealing the gas tax hike, while just 38 percent of registered voters supported keeping the higher taxes.

While the California GOP’s official stance is that Proposition 6 is meant to repeal an onerous tax on working Californians and hold Democratic lawmakers in Sacramento accountable for spending, some speculate that the initiative will also be used to bring GOP voters to the polls and thwart Democrats' efforts to win a handful of Republican-held seats across the state.

The repeal measure has garnered the support – and the donations – of numerous big-name Republican leaders in Washington, including outgoing Speaker of the House Paul Ryan, R-Wis., House Majority Leader Kevin McCarthy, R- Calif., and House Majority Whip Steve Scalise, R-La.

Jessica Levinson, a professor at Loyola Law School in Los Angeles, points out that California indeed has a “robust” history of ballot initiatives being used to significantly impact elections.

“It’s an effective way to connect voter issues with candidates,” Levinson, who specializes in ballot initiatives, told Fox News. “It games the electoral system in so many ways.”

The Democratic defections over to the side of Proposition 6 come not only as more and more Californians voice their displeasure at the gas tax, but as the party has already faced setbacks over the gas tax hike.

In June, Democratic state Sen. Josh Newman was recalled from office over his support for the tax hike.

“Pragmatically, candidates may be seeing opinion research that indicates the race is close, and opposition to the gas-tax repeal might cost them critically needed votes,” Darry Sragow, a longtime Democratic strategist and publisher of California Target Book, told the Los Angeles Times.

As a caveat, however, Sragow added, “Ideologically, they may have a heartfelt objection to the impact of the increase in the gas tax on people who survive from paycheck to paycheck and need to drive a car to make a living.”