Here’s why I think this is really important. The modern presidency comes with extraordinary power. The president has power over the entire executive branch, from the military to the Justice Department to the FBI to the administrative state. The Constitution provides for a number of checks on that power. The courts provide some very important checks, of course, at least when cases get to them.
But the most important checks on a president come from Congress. On a routine basis, the president needs Congress to enact laws that allow and pay for what the president wants to do. The president needs the Senate to confirm nominees. And at the very extreme end, the president needs to avoid an outright rebellion in Congress that could lead to impeachment and removal. As a result, how far Trump can go depends a lot on how far Congress will let him go.