President could even defy

top court on a subpoena

Suppose that Robert Mueller subpoenaed President Trump to compel him to testify before a federal grand jury and the President refused, with Mueller then taking the case to the Supreme Court.

Even if the top court sided with Mueller, the President -- under the Constitution -- could ignore the subpoena and the Supreme Court. The court could not authorize an arrest warrant of the President. In fact, the court could do nothing to enforce its ruling.

The only remedy would be for Congress to impeach and remove the President. But it is doubtful that a President's defiance of a subpoena would rise to the level of "high crimes and misdemeanors." Of course, if Congress were sufficiently incensed, it would find some grounds for impeachment. Yet it is quite possible that Congress would not wish to act.