Now a foreboding moment looms for his presidency and for the nation.

A stunning barrage of revelations on Tuesday suggested that at least one strand of Mueller's Russia probe is racing toward its end game, emphasizing the gravity of the situation facing the White House and the potential vulnerability of the President.

"It's possible that Mueller is closing in on his determination about what obstruction looks like, whether it is a criminal offense in his mind, whether it is an impeachable offense, or whether it amounts to nothing," Michael Zeldin, a former senior aide to Mueller at the Justice Department, told CNN's Brooke Baldwin.

Tuesday's bombshells came amid new signs that Trump is still pushing up against constitutional norms in his conduct toward judicial authorities usually seen as independent of the President.

The current FBI Director Christopher Wray threatened to quit, a source familiar with the situation told CNN, after coming under pressure from Sessions, to clean out senior leadership figures dating from the Comey era who the President believes are biased against him. The development was first reported by Axios.

And there are signs Rick Gates, the former Trump campaign staffer who pleaded not guilty in October to eight charges of money laundering and failing to register foreign lobbying and other business, may be ready to cooperate with Mueller, CNN's Katelyn Polantz reported Tuesday night

And on a day of intense drama, efforts by Republicans to discredit the Russia probe gathered pace, as the White House said Trump was ready to declassify a memo written by GOP committee staff in the House claiming misconduct by FBI officials investigating the President.

Four Trump associates have so far been charged in the Mueller investigation, but there is still neither proof of wrongdoing by the President nor indications of the special counsel's eventual conclusions.

Yet the prospect of the President of the United States testifying to the investigation would lift the intrigue to an unprecedented level, and would be a spectacle not seen since Bill Clinton's grand jury appearance 20 years ago that led to his eventual impeachment over an affair with White House intern Monica Lewinsky.

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The fact that Mueller's team has already spoken to Comey and Sessions and now wants to talk to the President suggests the investigation into whether Trump obstructed justice by asking the former FBI director to go easy on former national security adviser Michael Flynn and then fired him when he demurred, is at an advanced stage. CNN also reported last week that Trump's former top political adviser Steve Bannon had struck a deal to be interviewed by Mueller's prosecutors.

"It seems to indicate that the investigation is in its 11th hour," said Jens David Ohlin, a professor and vice dean at Cornell Law School.

Ohlin said the seniority of those questioned points to Mueller reaching a defining moment at least in the obstruction of justice portion of an investigation that is also considering whether anyone in the Trump campaign broke the law by cooperating with a Russian election meddling effort.

"We are at the top of the heap politically with Steve Bannon. We are at the top of the heap in terms of law enforcement in terms of the attorney general and the former head of the FBI," said Ken Cuccinelli, Virginia's former Republican attorney general.

"It strikes me they are getting near the end of gathering all their information, and then ... they are going to have to process that and make some decisions about how to proceed," Cuccinelli told CNN.

The interview request suggests that Mueller is proceeding in classic fashion, working his way from the broadest point of the investigation to the key figure, with Trump at the tip of the triangle.

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Trump has consistently denied any collusion with Russia, has said he did not obstruct justice in firing Comey and has branded the entire episode a "hoax" and a "witch hunt" against him.

The fact that Mueller has come this far does suggest that he believes at the very least there is "something there" in the possible obstruction case, Ohlin said: "I don't think it is just out of an abundance of diligence. He's doing it because he sees something problematic with Comey's dismissal. If there is truly nothing there, it is very easy to investigate it because there is just nothing to grasp onto."

The latest reports shed new light on Trump's potential exposure, noting that the special counsel's office has indicated it wants to talk to Trump about Flynn's exit and Comey's dismissal.

The Washington Post also said Mueller was interested in Trump's efforts to remove Sessions or to pressure him into quitting, quoting a source familiar with the investigation as saying the special counsel wanted to assess whether there was a "pattern" of behavior by the President.

Still, as Mueller has shown -- for instance in surprise indictments and plea deals last year -- no one outside his investigation can accurately assess his intentions or its scope.

"We also don't know if it will lead to anything at all. Sometimes you get to the end of an investigation and you have the final interview to check the box," former US Attorney Preet Bharara said on the "Situation Room" on Tuesday. "It may be that this will lead to something very significant and earth-shattering and earth-shaking for the country, it also could be something they are just putting to bed."

Trump's lawyers could offer written answers from the President to the special counsel's questions -- a step many legal analysts suspect will not go far enough for Mueller. Another option might be a formal interview with Trump's lawyers present, which would allow them to restrain their star witness, who is often voluble and imprecise and economical with the truth in a way that could lead him into jeopardy from Mueller's crack team of prosecutors.

Refusing Mueller's request could force the special counsel to subpoena the President to testify to a grand jury, a step that would put him in an even more perilous situation.

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Tuesday's developments also shed new light on Trump's potential vulnerability. Mueller is now armed with the testimony of Comey, Sessions and that of a cooperating witness Flynn, all of whom could have shed light on crucial conversations with the President.

He likely also has a large quantity of evidence from other sources, including interviews, emails and other testimony and doubtlessly already knows the answer to many of the questions he will pose to Trump, putting the President on thin ice if he is not completely truthful.

Mueller also has the benefit of contemporaneous memos written by Comey after several meetings with the President in which he detailed what he later said was an effort by the President to develop a inappropriate relationship of "patronage" with him.