Trump asked McCabe about his personal politics

Finally, inexorably, special counsel Robert Mueller's investigation has clawed all the way up to Donald Trump himself.

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.

Mueller's request to question Trump, and news that his team has already interviewed fired FBI Director James Comey and Attorney General Jeff Sessions, indicate that the special counsel has a clear picture of where he is headed in what could turn into an obstruction of justice case, legal experts said.

"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.

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 , 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.The Washington Post reported that Trump had asked then acting FBI director Andrew McCabe who he voted for in the 2016 election in an introductory Oval Office meeting and criticized his wife's Democratic affiliation, in a move that infringed customary treatment of a civil servant.

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.

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.