Since When Are Congressional Subpoenas Optional?

SEE HERE The executive branch is looking more and more like it imagines itself an imperial agency.  The president questions the legitimacy of the Supreme Court and his Interior Secretary takes it upon himself to ignore congressional subpoenas designating them "political" I suppose because the House has a Republican majority.  Does that mean that all the Democrat Senate's obstructionism is also just "political"?  The lack of respect for the Constitution that is reflected in the administration is calling into question the fundamental rule of law in the United States.