A day after Gov. Gavin Newsom held a splashy campaign rally to debut his ballot measure to redraw California’s congressional map, Arnold Schwarzenegger walked into a Santa Monica hotel for breakfast on Friday.

He was wearing a new custom-made T-shirt. It was emblazoned with an image of a raised fist, an expletive aimed at politicians and the phrase, “Terminate Gerrymandering.”

As governor of California from 2003 to 2011, Mr. Schwarzenegger led the charge to do just that. He fought to overhaul how the state draws political maps, ultimately winning when voters passed a pair of ballot measures that took that power away from politicians and gave it to an independent commission.

Now, Mr. Newsom is asking voters to set the independent commission’s work aside for the next three elections in favor of a map drawn to help elect more Democrats. He’s pitching it as a temporary pause on California’s bipartisan system that’s necessary to counter a Republican gerrymander President Trump is seeking in Texas.

And Mr. Schwarzenegger, a moderate Republican, finds himself fighting to preserve a key plank of his legacy as governor, a reform that has allowed what he calls his post-partisan style of politics to endure in California even as a brawling hyperpartisanship has become the national norm.

“I hate the idea of the Republicans redrawing the district lines in Texas, as much as I hate what the Californians are trying to do,” Mr. Schwarzenegger said in an interview at the Fairmont Miramar Hotel and Bungalows.