In summary Katie Porter’s announcement in a new campaign ad marks the most prominent defection over the gas tax repeal from the Democratic ranks yet.

The Republican-led campaign to repeal a recent increase in the state gas tax has a new high-profile backer: “Elizabeth Warren’s protégée.”

In a new campaign ad, Katie Porter, the UC Irvine law professor and progressive Democrat hoping to unseat Republican U.S. Rep. Mimi Walters in Orange County, made her position clear.

“I oppose higher gas taxes and I won’t be afraid to take on leaders of both political parties,” she said in the new spot, the first of the general election season from her campaign.

This November, California voters will decide whether to roll back a 12-cent hike in the state gas tax, along with other new fuel and vehicle fees. The tax increases were levied last year by a mostly Democratic coalition of state lawmakers in order to fund an extra $5 billion of annual transportation spending.

Though Porter does not mention Proposition 6 explicitly, voters should consider it an endorsement of the ballot initiative, said her campaign manager Erica Kwiatkowski.

“This is about fighting for the people you represent and Katie is running to represent the needs of the 45th district,” she said in an email.

The political battle over the gas tax has become one of the central debates of the California campaign season. In June, voters in north Orange County recalled Democratic state Sen. Josh Newman over his support for the gas tax increase. National Republicans—hoping that Prop. 6 will stir up GOP voters and mute a widely anticipated Democratic “blue wave” that could cost Republicans control of the House—have piled on. The political committee for House Speaker Paul Ryan has put $50,000 behind the repeal effort. Walters has donated $110,000 from her own campaign coffers.

In this fiercely partisan debate over Prop. 6, Porter’s announcement marks the most prominent defection from the Democratic ranks yet. (It also may mark the first time that the anti-tax Howard Jarvis Taxpayers Association and a single-payer advocate have stood on the same side of a tax policy debate.)

Porter, who studied under Sen. Warren at Harvard and touted the endorsement of the progressive senator throughout the primary, now finds herself in odd company. Prop. 6 is backed by John Cox, the conservative businessman running for governor, along with the rest of the California Republican Party. They argue that the state should spend its transportation dollars more efficiently and not place a greater financial burden on drivers.

Opponents of the gas tax repeal include Democratic Lt. Gov. Gavin Newsom and the California Democratic Party, along with a broad coalition of labor, environmental, and major business groups who argue that the state’s crumbling roads and highways desperately need the extra funding.

Porter’s embrace of Prop 6 comes in response to an attack ad released last week by the GOP-affiliated Congressional Leadership Fund, which criticized Porter for not opposing the gas tax.