A proposal to raise taxes on the wealthiest Californians has sharply divided Democrats in the biggest blue state in the country — and is poised to set up a broader debate for the party heading into the next presidential contest. Clear battle lines have formed in recent weeks among the sprawling field of Democratic candidates in California’s open governor’s race this year, as well as among potential 2028 White House contenders in the state, over a “billionaire tax” that could appear before