Gov. Gavin Newsom vowed on Monday to stop a proposed wealth tax in California, saying that its mere introduction had already hurt the state by driving some billionaires to relocate and take their tax dollars with them.

Mr. Newsom, a Democrat, said in an interview with The New York Times that he had been relentlessly working behind the scenes against the proposal, and that he would fight the measure if it reached the November ballot.

(snip)In past years, he has quashed legislative wealth tax proposals by making it clear that he would not sign them into law. The issue has urgency now, though, because a large health care union is trying to place a new billionaire tax on the ballot.

The initiative would require Californians with a net worth beyond $1 billion to pay a one-time tax equal to 5 percent of their assets. It would apply retroactively to anyone who was living in California as of Jan. 1, and taxpayers could spread their payments across five years starting in 2027.