Governor Gavin Newsom of California is vowing to block a proposal for a 5 percent, one-time tax on the state’s billionaires. By bucking Democrats on an article of left-wing orthodoxy, their potential 2028 presidential candidate is previewing a strategy to capture the White House by staking out middle ground between political extremes.

“Run to the right in the primary election,” President Nixon advised GOP candidates, “and then run to the center in the general election.” Mr. Newsom is laying the groundwork for that shift from the left early. He’s banking that his lead in the Democratic field will hold.

In an interview Monday with Politico, Mr. Newsom cited a flurry of headlines about billionaires getting ready to pull up stakes if the proposed tax becomes law. “This is my fear,” he told the wire. “It’s just what I warned against. It’s happening.”

The “2026 Billionaire Tax Act” is backed by the Service Employees International Union-United Healthcare Workers West. The unions are gathering the about 900,000 signatures needed to put it before Californians and make those with net worths over $1 billion surrender 5 percent of their assets.

Anyone who was a resident retroactive to January 1 would be on the hook beginning in 2027 and have five years to pay. Mr. Newsom has long opposed such measures. He told Politico that California is “in a competitive reality with 49 other states,” and can’t price out residency.

Mayor Michael Bloomberg of New York City argued in 2012 against extending a “millionaire’s tax” on the same grounds. “The wealthy,” he told WOR Radio, “are mobile.” California already has the nation’s top marginal income tax rates and, according to the U-Haul Growth Index, led the nation again last year in outgoing moves.

The Golden State is home to the most billionaires in America, with data compiled by Americans for Tax Fairness pegging their number at 214. Even a few of them leaving would erode the tax base, as the Legislative Analyst’s Office of California stated in an analysis released last month.

The Analyst’s Office wrote that the tax “probably would collect tens of billions of dollars.” It’s “likely that some billionaires” would flee and take what “they currently pay” with them. After Norway imposed a wealth tax, as this columnist wrote for the Sun in July 2023, so many fled that it resulted in a net loss of $448 million.

On Wednesday, analysis by the Tax Foundation predicted that the Billionaire’s Tax would catch people of more modest means, too. Under the “poorly drafted” proposal, they wrote, “valuation can be based on voting interests in a company when they exceed actual economic stakes” and assessments could “substantially overvalue” businesses.

These arguments will be touted as evidence that Mr. Newsom isn’t just another tax-and-spend liberal out to punish the rich. In this way, it echoes the “Triangulation” strategy that President Clinton’s adviser, Doug Schoen, pioneered in the 1992 presidential campaign.

Mr. Schoen identified issues where Mr. Clinton could position himself as a moderate. “Personal responsibility” was a phrase that Mr. Schoen encouraged the then-governor to use often. A balanced budget and middle-class tax cut were other signature issues.

As president, Mr. Clinton abandoned the tax cut, framing the move as the price of putting the nation’s finances in order. Mr. Newsom is betting that Democrats today will accept his opposition to taxing billionaires because, as with Mr. Clinton, they want a winner. They may also assume that he, too, will return to the fold once in the White House.

Working against the regional union backing the Billionaire’s Tax will also help Mr. Newsom appeal to moderates. It will counter the charge that he’s beholden to Big Labor while pleasing wealthy donors and businesses who are loath to have 5 percent of their assets confiscated.

Opposition to the Billionaire’s Tax is a solid strategy for the general election. To get there, Mr. Newsom first has to overcome more liberal candidates in the primaries. That gauntlet will test whether applying Nixon’s advice in reverse works, or if the governor is turning to the center too soon and would have been better off sticking closer to his base.