Sen. Richard Blumenthal (D-CT) just called on Robert Mueller to subpoena Trump, and if he refuses to comply, take him to court.
Trump's lawyers are trying to muddy up two questions that already have legal opinions behind them. The DOJ determined previously that Nixon couldn't pardon himself, and there are numerous court rulings that suggest that a president must comply with a subpoena.
Robert Mueller isn't playing. As the Trump team was engaged in tense discussions with the Special Counsel, they tried to claim that the president didn't have to speak to Mueller, the Special Counsel replied that he could subpoena the president.