On Friday morning, President Donald Trump announced that he would be declaring a state of emergency on the US-Mexico border and unilaterally appropriating funds to pay for his border wall.

It’s not clear if he can actually do that: The Constitution gives Congress, not the president, the ultimate power to appropriate money. The president legally has the power to declare emergencies and respond, but can he do that in a situation where Congress has explicitly declined to fund the president’s wall?

According to Elizabeth Goitein, an expert on national security law, the answer is that he can’t — and Trump’s attempt to do so constitutes a “constitutional crisis.”

Goitein is the co-director of the Liberty and National Security Program at the Brennan Center for Justice, a nonpartisan but liberal-leaning legal nonprofit. Her research focuses on balancing national security and constitutional rights, which makes her pretty well-positioned to evaluate the president’s claim. In a series of tweets, she made the case that declaring an emergency on the border constitutes a power grab that directly threatens the constitutional order.

Here’s the argument, which focuses not only on Trump but on the underlying laws that enable him to declare an emergency in the first place:

Today, the President of the United States will declare a fictitious national emergency for the sole purpose of giving himself powers Congress explicitly refused to provide. 1/13 — Elizabeth Goitein (@LizaGoitein) February 15, 2019

This is a constitutional crisis. Article I expressly forbids spending money except as appropriated by Congress. Congress had repeatedly refused to provide funds to build the wall. 2/13 — Elizabeth Goitein (@LizaGoitein) February 15, 2019

As I wrote in @TheAtlantic, emergency powers are not, and were never intended to be, a constitutional workaround for a president who cannot bend Congress to his will. 3/13 https://t.co/MpKVec800V — Elizabeth Goitein (@LizaGoitein) February 15, 2019

Emergency powers are designed to give the president access to standby authorities, passed by Congress in advance, in situations where Congress has no time to act. 4/13 — Elizabeth Goitein (@LizaGoitein) February 15, 2019

If Congress *does* have time to act, then there’s no justification for bypassing the ordinary legislative process. And when that legislative process yields a clear answer, as it did here, the Constitution commands the president to respect that answer. 5/13 — Elizabeth Goitein (@LizaGoitein) February 15, 2019

I hope the courts do the right thing and put an end to this abuse of power. But whether they do or not, it’s time for Congress to revisit the current legal system for emergency powers. 6/13 — Elizabeth Goitein (@LizaGoitein) February 15, 2019

Under the National Emergencies Act, it’s far too easy for a president to declare emergencies where none exist—and far too difficult for Congress to put a stop to them. There’s no definition of “national emergency” and no limits on how many times a president can renew them. 7/13 — Elizabeth Goitein (@LizaGoitein) February 15, 2019

If Congress believes the president is misusing emergency powers, its only remedy is to pass a law that the president has to sign. In practice, that means Congress needs a veto-proof majority. 8/13 — Elizabeth Goitein (@LizaGoitein) February 15, 2019

Once an emergency is declared, the president has access to 123 special provisions of law that give him extraordinary powers—as we found in our research, available here: https://t.co/PIHvDTpw10 @BrennanCenter 9/13 — Elizabeth Goitein (@LizaGoitein) February 15, 2019

This system invites abuse. Presidents need broad discretion to decide what constitutes an emergency, but that discretion shouldn’t be unlimited. Some basic, common-sense criteria are needed. 10/13 — Elizabeth Goitein (@LizaGoitein) February 15, 2019

At a minimum, an “emergency” should involve a significant change in factual circumstances that poses an imminent threat to public safety or other important national interests. That would still give the president plenty of wiggle room, without allowing fake emergencies. 11/13 — Elizabeth Goitein (@LizaGoitein) February 15, 2019

And the president should not be able to renew states of emergency indefinitely. After a short period (I agree with @steve_vladeck that 30 days should be sufficient), the emergency should expire unless *Congress* renews it. 12/13 — Elizabeth Goitein (@LizaGoitein) February 15, 2019

Congress should act *now* to try to block this abuse of power, and the courts should play their constitutional role. But unless we want to see more of this kind of abuse in the future, we need to get serious about National Emergencies Act reform. 13/13 — Elizabeth Goitein (@LizaGoitein) February 15, 2019

One thing is practically certain: Goitein’s arguments will be tested in court. Someone will try to sue Trump to stop the emergency declaration; House Democrats already have a plan for a legal challenge, per the Washington Post. Then it’ll be up to the courts to decide if she’s right.