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Xavier Becerra, California’s attorney general, has already sued the Trump administration more than 40 times, positioning himself on the front lines of the Golden State’s resistance to the president’s policies.

Yesterday, he added another big lawsuit to the tally.

Mr. Becerra, along with his counterparts in 15 other states, challenged President Trump’s plan to declare a national emergency in order to access billions more dollars for a border wall than Congress had granted him.

“President Trump treats the rule of law with utter contempt,” Mr. Becerra said in a statement announcing that the suit had been filed in Federal District Court in San Francisco. “He knows there is no border crisis, he knows his emergency declaration is unwarranted, and he admits that he will likely lose this case in court.”

It’s a fight that could have major constitutional ramifications. And California is playing a central role.