Trump broke the law. Congress must now defend the separation of powers.

On Thursday morning, hours before senators were sworn in to serve on President Trump’s impeachment trial, an independent, nonpartisan government watchdog confirmed what I have long suspected: When Trump froze congressionally appropriated military aid to Ukraine as part of an effort to compel Ukraine to investigate his political rival, he broke the law.

That’s because a central feature of our republic, defined by its separation of powers among the three branches of government, is that Congress, not the president, controls the “power of the purse.” James Madison argued that this was “the most complete and effectual weapon” to counter “all the overgrown prerogatives of the other branches.” The nation’s founders enshrined their vision in Article 1, Section 9 of the Constitution, establishing that Congress alone possesses the power of the purse. The president can propose funding for whatever projects he wants, but Congress ultimately decides where to direct the American people’s tax dollars.

Nearly 200 years later, in 1974, Congress overwhelmingly enacted the Impoundment Control Act (ICA) in response to President Richard M. Nixon’s overreach into Congress’s appropriations power. The ICA grants presidents limited ability to cut or delay any spending appropriated by Congress. If presidents wish not to spend appropriated money, they must seek and obtain Congress’s approval. Even a delay in spending requires notice to Congress.