How far left is the Democratic Party headed? Look to California, where last month’s state party convention went off the deep end.

The first sign of lunacy was the delegates’ refusal to endorse impeccably liberal Sen. Dianne Feinstein.

She lost out by 37 percent to 54 percent to Kevin de Leon, president pro tempore of the state Senate, who promises mainly to be obsessively anti-Trump.

Similarly, the delegates put former Los Angeles Mayor Antonio Villaraigosa last among the party’s four candidates for governor — even though he’s tied in polls of Democratic voters with former San Francisco Mayor Gavin Newsom, who came out on top at the convention.

Villaraigosa’s sin? As mayor, he supported charter schools and jousted with the teachers unions, rightly calling them “the largest obstacle to creating quality schools.”

Finally, the convention took a sharp anti-Israel turn, adopting planks in support of the Boycott, Divest and Sanctions movement and slapping the Israel Defense Forces.

Historically, the Golden State has been a harbinger of national trends. If that holds true, the whole Democratic Party is headed right . . . er, left . . . off the cliff.