Sixteen US states sued President Donald Trump’s administration on Monday over his decision to declare a national emergency to fund a wall on the southern border with Mexico, saying the move violated the constitution.

Trump announced the emergency on Friday in order to bypass Congress, which approved only a quarter of the $5.6-billion he wanted for the wall in a spending bill.

But the lawsuit, filed in a federal court in California, said the president’s order was contrary to the Constitution’s presentment and appropriations clauses, which outline legislative procedures and define Congress as the final arbiter of public funds, respectively.

The move had already been announced by Xavier Becerra, the attorney general of California, who said on Sunday his state and others had legal standing because they risked losing moneys intended for military projects, disaster assistance and other purposes.

Critics, including several senators from Trump’s Republican party, have warned that he has opened the door for future presidents to call on the act whenever they fail to get their way with Congress.

Should the states prevail, the case could work its way up to the Supreme Court, setting up a precedent-setting showdown on the separation of powers.

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