Sixteen states filed a federal lawsuit on Monday challenging President Donald Trump’s national emergency declaration to fund a wall along the U.S.-Mexico border.

Trump declared a state of emergency along the southern border Friday, which frees up federal funding and circumvents the budget deal Congress passed to avoid a second government shutdown, which allocated nearly $1.4 billion to border fencing. Trump’s emergency order would give him $6.7 billion more, the Guardian reports.

The suit, filed in U.S. District Court in San Francisco, argues that Trump does not have the power to divert funds for constructing a border wall because Congress controls spending.

“We’re suing President Trump to stop him from unilaterally robbing taxpayer funds lawfully set aside by Congress for the people of our states. For most of us, the Office of the Presidency is not a place for theater,” California Attorney General Xavier Becerra said in a statement.

Trump slammed California in a tweet Tuesday morning in response to the lawsuit, saying, “As I predicted, 16 states, led mostly by Open Border Democrats and the Radical Left, have filed a lawsuit in, of course, the 9th Circuit! California, the state that has wasted billions of dollars on their out of control Fast Train, with no hope of completion, seems in charge!”

Presidents have invoked their emergency powers nearly 60 times since Congress enacted the National Emergencies Act of 1976, the New York Times reports, but never before has it been used to bypass Congress after specific project funding was rejected.

Who’s behind the lawsuit?

California led the lawsuit against Trump and is joined by Colorado, Connecticut, Delaware, Hawaii, Illinois, Maine, Maryland, Michigan, Minnesota, Nevada, New Jersey, New Mexico, New York, Oregon and Virginia — all of which have Democratic attorneys general and all but one of which are led by Democratic governors, the Wall Street Journal points out.

Where’s that $8 billion coming from?

In addition to the $1.4 billion approved by Congress for border fencing, the emergency declaration allows Trump to redirect $3.5 billion from the Defense Department’s military construction budget. Trump said that projects the Pentagon intended to pursue with the construction money “didn’t sound too important to me.”

Trump will also use ordinary executive authority to redirect $2.5 billion from the Defense Department’s drug interdiction efforts and $600 million from the Treasury Department’s drug forfeiture program.

What happens next?

The states’ lawsuit is likely to stall the emergency declaration and kick off protracted legal battles that could land before the Supreme Court. The case may not be resolved before 2020, bringing the border wall issue into the 2020 election season.

At least two more lawsuits are expected to be filed later this week, the New York Times says. The American Civil Liberties Union has said it will file a case but has not yet publicly identified its client.

Protect Democracy, a watchdog group, and center-right policy institute the Niskanen Center, will file a suit on behalf of El Paso County and the Border Network for Human Rights. House Democratic leaders are considering filing a suit of their own, the WSJ reports.

Could the courts limit Trump’s emergency powers?

Courts have limited the president’s emergency powers before, including in a 1952 Supreme Court decision that said President Harry Truman could not use an emergency declaration to seize steel factories to ensure production during the Korean War. Laws governing emergency authority have changed since then, however, and it’s difficult to predict how a case would be viewed by the current Supreme Court, in which Republican-appointed justices hold the majority.

The Trump administration has a powerful argument on their side, NYT reports: In the National Emergencies Act, Congress defined no standard for what conditions have to be met for a president to declare a crisis. But Becerra said that the president himself had undercut his argument that there was an emergency on the border, referring to Trump’s speech on Feb. 15 announcing the plan: “I didn’t need to do this, but I’d rather do it much faster.”