Now to the showdown tonight between president trump and the special counsel, Robert Mueller. Mueller is now threatening a subpoena if the president is unwilling to answer questions in the Russia investigation. And tonight, white house lawyer Ty Cobb announcing he's retiring at the end of the month. Here's ABC's chief white house correspondent Jonathan Karl. Reporter: When we first asked, the president was adamant. He'd be more than happy to talk to special counsel Robert Mueller. Would you be willing to speak under oath to give your version of those events? 100%. Reporter: But now, nearly a year later, a tense standoff. So tense, ABC news has confirmed Mueller told the president's lawyers he could compel the president to testify. Raising the possibility of a subpoena. John dowd, until recently the president's lead lawyer, tells ABC news he fired back, respond, quote, "This isn't some game. You are screwing with the work of the president of the united States." Dowd has since left the president's legal taerjs and today, another shakeup. Out, Ty Cobb, the lead white house lawyer dealing with the special counsel, replaced by emmet flood, who was one of president Clinton's impeachment lawyers. Cobb tells me he was ready to leave. Almost everything that I was brought on to do has been accomplished. Reporter: Cobb had employed a strategy of patience with Mueller. You've been preaching cooperation, is that going to change? Cooperation was definitely the right move and definitely the path that would lead to the quickest resolution. So, I hope that they're able to work something out. Reporter: Today, Rudy Giuliani, who is leading the negotiations with Mueller, says whether a presidential interview happens depends largely on the special counsel. Do they have an open mind to the fact that he may be telling the truth and Comey may be lying? If they have an open mind to that, then this is something we would consider. If they don't, we would be foolish to have him be interviewed. Reporter: Giuliani told the Washington post today that any interview must be limited in scope. "Max, two to three hours around a narrow set of questions." So, let's bring in Jon Karl, live from the white house tonight. And Jon, president trump now threatening to get involved with the justice department if they don't provide Republicans with documents they're demanding in congress, saying this is a rigged system. But Jon, bottom line, in the showdown with Mueller tonight, even if the president is subpoenaed, does he have to answer those questions? Reporter: Well, he could fight that subpoena, David. It's a fight that would almost certainly go all the way to the supreme court, and it would be a tough one to win. When Richard Nixon tried to fight the subpoena for his oval office tapes, he lost at the supreme court, 9-0, so, for now, the president's legal team is trying to narrow the time and the scope of any interview, and David, they are also demanding to know the questions in advance. Jon Karl tonight. Jon, thank you, as always.