It's part of the Constitutional authority of Congress the Supreme Court has decided -- the power of Congress to demand documents and witnesses from the Executive branch. One can read up on that. Here's where I did:
Congressional oversight - Wikipedia
Reading this, one can see Congress has that Constitutional authority to require whatever officials/workers in the executive branch to appear before it and more: it also has the duty of oversight.
Both the Constitutional power, and the Constitutional duty.
So, to block that rightful power of Congress by refusing to allow witnesses to go to Congress is against the Constitution.
That blocking when it happens is by itself, alone, strictly speaking so serious it warrants the impeachment and removal of any such president breaking his oath to uphold the Constitution.
Regardless of any other factors.
(btw, I'm non partisan. But I do like to see large numbers of legit conservatives in office. We'd be far better off with Pence, at this point)