This week, the Government Accountability Office, essentially the federal government’s rule enforcer and auditor, located in the Legislative branch, put out a decision that the Trump administration violated the law when it withheld funds from Ukraine that Congress had appropriated for that country. The law that was broken is the Impoundment Control Act, which says that the president can only hold back funds if he or she asks Congress to do so. The law itself is very clear on how the president is supposed to inform Congress by a “special message” that includes several conditions. Donald Trump, of course, didn’t inform Congress at all.
By the way, the law was passed by overwhelming bipartisan margins in 1973 and 1974 before being signed by Richard Nixon, who was, you know, a Republican.