As Americans wait to hear precisely what President Trump will say during his trumpeted address to the nation at 9 p.m. tonight, speculation is continuing that he could declare a so-called “national emergency” concerning border security and use it to unilaterally order the military to build a border wall or fence across our southern border. This is despite the fact that Congress has repeatedly refused to appropriate the billions of dollars needed to pay for it.

Such a move by Trump would go further than he has ever before to seize and use power just like an authoritarian dictator. While we do not yet know precisely what alleged authority he will claim for his actions, it appears likely that such actions would violate federal law and the Constitution. And Congress, the courts, and the American people will need to act quickly in response.

Congress has given presidents the authority to declare national emergencies and take action under some circumstances, but such declarations have usually concerned actions like blocking transactions with foreign powers or responding to a natural disaster. But because it is unlikely that there is anywhere near the $5.7 billion needed for a wall already appropriated by Congress for the military and available, Trump would likely have to “usurp Congress’ power of the purse,” as House Judiciary Committee Chair Jerrold Nadler has explained, which would violate federal law and the Constitution. In addition, Nadler and others have pointed out, there is no true emergency at the border other than “the humanitarian one caused by this administration’s” actions.

Experts across the ideological spectrum have accordingly concluded that it would be illegal for Trump to declare a national emergency and unilaterally order the building of his wall. Yale Law professor Bruce Ackerman has written that not only would such an order by Trump be “illegal,” but also that members of the military who obeyed his commands “would be committing a federal crime.” Fox News’ Andrew Napolitano has concluded that Trump “can’t spend money and he can’t take property” to build the wall “unless Congress has authorized it;” otherwise, he would be acting like a “prince” and “not a president.”

To stop such improper conduct by Trump, whether declared tonight or later, immediate action is necessary by Congress, the courts, and the American people. Under the national emergency law that Trump may try to rely on, the House can immediately pass a resolution repudiating the President’s emergency declaration and send it on to the Senate, which is “required to conduct a floor vote within 15 days.” Congressional committees can and should demand testimony and documents from administration officials about the basis for the alleged emergency and the specifics of the wall. And members of Congress should be prepared to serve as plaintiffs in court, challenging the president’s actions.

Such challenges in the courts are likely within days if Trump takes his threatened action. We do not know, of course, how the courts will rule, but there is reason for concern in light of the large number of federal judges Trump has placed on the bench, including two on the Supreme Court. But in 1952, when President Harry Truman tried to invoke a national emergency in connection with the Korean War to justify his seizure of steel mills, the Supreme Court voted 6-3 to reject his claim, including Justice Tom Clark, who had been appointed by President Truman and previously served as his Attorney General. Hopefully, our current Supreme Court will show similar independent judgment.

Perhaps most importantly, the American people must act if Trump behaves even more like a dictator in trying to order the building of his wall. Polls have shown that most Americans disapprove of the wall and Trump’s prompting of the current partial government shutdown. Americans must constantly urge members of Congress, both Republican and Democratic, to oppose Trump’s conduct, to condemn it from the floor of Congress, and to go to court if necessary. Only with such pressure from all of us can we hope not only to stop tyrannical action by Trump but also to push Trump and Congress to end the government shutdown.