Members of Congress have good cause to worry about the extent to which President Trump ignores legal constraints. But they also have given him plenty of authority to impose his will lawfully. Yesterday’s announcement of weapons sales to Saudi Arabia and other Middle East countries is illustrative. Even though the Constitution does not grant Presidents the power to make arms deals on their own, Congress has done so.

For the new arms sales, Trump invoked a provision of the Arms Export Control Act, which permits Presidents to act when “an emergency exists that requires the proposed sale in the national security interest of the United States.”

Of course, when Congress gave emergency powers to Presidents, it assumed that Presidents would use such powers sparingly and reasonably. But why make such an assumption? The Framers of the Constitution understood that people are not angels and that we can’t rely on the virtue of our elected officials. So the constitutional drafters gave us their careful system of checks and balances in which the executive and legislative branches would need to work with each other to pursue their policy preferences. Unfortunately, Congress has repeatedly given Presidents unilateral authority on many important policy decisions, thereby creating an executive power that can be easily misused.

Yesterday’s border wall decision by a federal court reminds us that Congress can give Presidents necessary authority without writing a blank check. As the court observed in rejecting President Trump’s effort to divert one source of Department of Defense funding to a border wall, the statutory provision authorizing such diversions explicitly withholds authority “where the item for which funds are requested has been denied by the Congress.” And Congress had refused Trump’s request for more border wall funding.

As Congress considers ways to restrain President Trump, it must take steps that will provide appropriate limits for future Presidents as well.