Can a POTUS defy a congressional subpoena?

I am believing we are about to find out in short order whether Congress has the stones to enforce a subpoena it would issue to the president of the United States.

Donald Trump has ordered a State Department official, Gordon Sondland — the U.S. ambassador to the European Union — to ignore a congressional summons to testify about what he knows regarding the, um, Ukraine Matter. Sondland took part in a conversation with other officials regarding the phone call Trump had with the Ukrainian president, the one that has gotten Trump into so much trouble; it’s the call in which he asked for a political favor in exchange for his releasing money to help the Ukrainians fight Russian-backed rebels with whom they are at war.

There appears to be a subpoena on the horizon, don’t you think?

So, what then? Does Congress issue the subpoena and then let the White House run roughshod over its constitutional authority? Hmm. I doubt it.

We now might have a case of the president piling yet another impeachable offense on those that already are building. You know, something about “no one being ‘above the law.'” If Donald Trump believes he is “above the law,” then it well might fall on the House to throw that count onto an article of impeachment resolution.

It’s getting weird.