Per the Constitution, “The executive power shall be vested in a President of the United States of America.” This is the basis of the unitary executive theory that many on the right (including Kavanaugh) argue in favor of. The truth is, there isn’t a lot of debate in the legal community as to whether we have a unitary executive, because the text is pretty clear that we do. All executive power essentially flows from the office of the president. The debate is mostly in the margins or when dealing with the administrative state.

Executive appointees serve, as the saying goes, at the pleasure of the president. Which is why the president can fire most appointees on a whim. There’s been a few instances when Congress has tried to limited that power - like the Tenure of Office Act which was the underlying issue in Johnson’s impeachment. It was eventually repealed and in the 1920’s when reviewing a case involving a similar law (dealing with the Postmaster General who was no longer considered a cabinet level official), the Supreme Court found the law in that case to be unconstitutional and suggested that the Tenure of Office Act was also unconstitutional.

Now, Congress can vest the appointment power in other officials within the executive branch. But ultimately the people making those decisions are still going to have to answer to the president, and you end up with a Saturday Night Massacre situation.

Nixon tried to end the investigation into his wrongdoing. And members of Congress finally told him it was enough, and if he didn’t resign, he’d be impeached and removed. The Independent Counsel law was created after Nixon resigned. Unfortunately, pretty much everyone hated that law once it went into effect - every president who served while the law was in place was subject to an investigation under it. That’s why Congress let it expire.

And the fact is, we’ve had other times in history when the president acted illegally and Congress went along with it. For example, despite a Supreme Court ruling against the removal of Native Americans from Florida, Jackson allegedly said, “Chief Justice Marshall has rendered his decision, now let him enforce it.”

Ultimately, if Congress is unwilling to serve as a check against the president, the president can do a lot of terrible shit. We’re only as good as our elected officials, and right now, that’s not great.