We begin with that stunning headline in the Russia investigation. Special counsel Robert Mueller has warned president trump's legal team that if trump refuses to an interview Mueller could subpoena him to testify before a grand jury. It is a dramatic threat that could lead to a showdown in the supreme court. Pierre Thomas starts us off, good morning, Pierre. Reporter: George, good morning. The negotiations between the special counsel and president trump's legal team about whether the president will sit down for an interview have become more tense recently. And if they completely fall apart, the specter of an all-out court battle now looms as a real possibility. ABC news has learned that special counsel Robert Mueller is considering subpoenaing president trump if he does not agree to a request for an interview. Sources confirming that Mueller told the president's legal team earlier this year that if they reach an impasse he might seek a subpoena to compel the president to testify before a grand jury. Negotiations between the two under way for months had stalled into the recent addition of Rudy Giuliani in his stable of attorneys. There won't be a stitch of evidence he colluded. Reporter: The prospect of a legal fight between the attorneys could lead to a fierce court battle. This coming as we learn that president trump's attorneys developed a list of questions based on conversations with Mueller's prosecutors concerning what may be asked. The questions include a number of critical topic, the firing of former FBI director James Comey, the firing of former national security adviser Michael Flynn. At issue whether there was obstruction of justice and collusion between trump associates and the Russians. President trump still calling it all a witch-hunt and the white house refusing to comment on the state of negotiations. I'm not going to get into a back and forth about questions leaked or anything having to do with the special counsel and I would refer you to the president's attorneys. How intense have these been? Response to that threat of a subpoena, John dowd, the president's former attorney told Mueller's team you're screwing with the work of the president of the United States. It is getting serious. Pierre, thanks very much. Let's bring in Dan Abrams with more. This is real right now. If this -- if president trump refuses to answer questions, if Robert Mueller then does subpoena him, it goes to the courts. The courts have decided on this before. Let's take one step back which is typically someone could go in and invoke their fifth amendment right and that's fine. We saw Michael Cohen do that. Exactly, exactly, but because he's the president and because there are political implications I don't think anyone expect president trump will go and show up and invoke his fifth amendment right. Two, what happens then? This fight begins and, you're right, two big cases with Richard Nixon not with regard to testimony but tapes, then you have the Paula Jones case, ultimately Bill Clinton and with Ken Starr where it didn't work its way up to this question but all the cases together would clearly indicate that the president does not have a winning legal argument that executive privilege. President Clinton did agree to answer some questions for the grand jury. President trump's team seems to suggest they don't want to answer the questions right now. If the court rules against him, that's when it gets exceedingly serious. Let's say it works its way through the court and the supreme court says it's a valid subpoena and the president says I don't think so and I'm not going to show up. That's when you have the real potential crisis. I think the only real crisis occurs if you have in the end a real war between two branches of government where one is saying, you can't tell me what to do. Even though it's always been accepted that the courts are the final arbiters on a question. That's why you believe it's possible Mueller wouldn't follow through. I still don't think Mueller will do it. There is a difference between will and could. I could and I will is different. I don't think he will do even if trump and the Mueller team aren't able to reach common ground on the questions. A lot left to play out. Thanks very much.