This story was updated on January 27, 2020 at 11:00am.

The Senate impeachment trial of President Donald Trump is under way. Senators now face the monumental question of whether to remove the president from office—and yet, something bigger is at stake here. The Constitution’s fundamental design is on trial too.

This is clear from the articles of impeachment themselves. Start with the first article, which charges that the president abused his power by holding support for an ally foreign government hostage to better his chances at reelection, rather than advancing the interests of the United States.

But President Trump, with the support of his attorneys at the Department of Justice and his own White House counsel, has argued that he has absolute authority over foreign policy. Thus, no abuse of power is possible, because any power he exercises is legitimate by definition. His attorney Alan Dershowitz has told the public he will go so far as to argue that the Constitution does not even allow impeachment for abuse of power, only for crimes.

Trump’s defense is a direct test of the Constitution’s allocation of power when it comes to foreign policy. The Constitution distributes foreign-policy power between Congress and the executive branch. The president commands the armed forces and controls diplomacy through the State Department, but Congress raises and pays for the military, appropriates foreign aid, and has the sole power to declare war.