Paul Callan is a CNN legal analyst, a former New York homicide prosecutor and currently is of counsel at the New York law firm of Edelman & Edelman PC , focusing on wrongful conviction and civil rights cases. Follow him on Twitter @paulcallan . The opinions expressed in this commentary are his own.

(CNN) The President's latest Twitter attack on the special counsel investigation by Robert Mueller is likely to claim a new victim: Deputy Attorney General Rod Rosenstein, the lawyer at least nominally in charge of the investigation.

Rosenstein was the Justice Department official who drafted a memo to the President alleging numerous deficiencies in the performance of then-FBI Director James Comey. The memo included stinging criticism of the FBI director's handling of the investigation of Hillary Clinton's email.

Shortly thereafter, the President shocked the nation with the announcement that he had fired the controversial FBI director he had only recently praised. The White House press office immediately pointed to Rosenstein's detailed memo as the justification for Comey's surprise termination.

Later, that position was contradicted by the President himself, in an interview with NBC's Lester Holt, when he stated that he had made up his mind to fire Comey before he had even seen the Rosenstein memo. In the same interview the President suggested that he was thinking about the Russia investigation at the time he terminated Comey.

On the following day in an Oval Office meeting with Russian officials, the President described his FBI chief as a "nut job" and "crazy" and expressed relief that the "pressure " of the Russia investigation had been lifted by the firing of Comey.

In a move that surprised and angered the President, new Attorney General Jeff Sessions, an early and ardent supporter off the Trump campaign, had recused himself from any aspect of the FBI's then ongoing investigation into the Russian alleged hacking of the presidential election and whether any Trump campaign officials colluded with the Russians in their anti-Clinton efforts.

Sessions quite rightly decided that he had a conflict of interest, since his role in the Trump campaign might be scrutinized as a part of the FBI's investigation. This would necessarily occur, even though Sessions had repeatedly denied improper or illegal contact with the Russians. It would be wrong for Sessions to exercise a supervisory role in any investigation that focused even in part on him.

Thus, the responsibility of supervising the Russia investigation fell to the highly respected Rod Rosenstein, who had been serving as acting attorney general prior to Sessions' Senate confirmation. Rosenstein then surprised the President by announcing that potential Justice Department conflicts of interest relating to the Russia Investigation required the appointment of the respected and independent Robert Mueller as a special counsel to supervise the FBI's now rudderless investigation.

Rosenstein made it clear in public statements and in congressional testimony that the new special counsel, though tasked with the Russia investigation, could follow the evidence where it took him if related to that probe.

Mueller was immediately confronted with the reality of Trump's Lester Holt interview, his discussion with Russian officials in the White House regarding his relief that the firing of his "crazy" FBI Director Comey would take the pressure off the Russian probe, Comey's testimony about Trump asking for his "loyalty" pledge, and what Comey perceived as the President's directive (the President used the word "hope") to terminate the Michael Flynn investigation.

In light of these facts, Democrats and many in the media across the nation called for an obstruction of justice investigation of the president.

Mueller had no choice but to add obstruction of justice to the list of potential issues his office would investigate.

All of which necessarily brings us back to Rod Rosenstein. A Trump tweet Friday morning asserted:

"I am being investigated for firing the FBI Director by the man who told me to fire the FBI Director! Witch Hunt." This was clearly a reference to Rosenstein, who as the current deputy attorney general is in overall charge of Mueller's special counsel probe.

The President, perhaps unintentionally, has focused attention on a new and serious problem with the Mueller probe.

If there is an obstruction case against the President, one of the chief claims would be that Rosenstein's memo was a ruse orchestrated to shift attention away from the real reason for the Comey firing: the president's opposition to the FBI's investigation of whether Trump campaign aides colluded with the Russians to influence the presidential election campaign.

Rosenstein will have to recuse himself from any supervisory role in the Mueller investigation due to this possible conflict of interest. He may actually be a witness in the investigation he is supposed to supervise and like Sessions will be compelled by legal ethics to assume a seat on the sidelines. A Justice Department spokesman didn't rule out that possibility Friday, saying, "As the Deputy Attorney General has said numerous times, if there comes a point where he needs to recuse, he will."

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Like her famous predecessor as a high-ranking Justice official, the Nixon administration's Robert Bork who played a key role in the "Saturday Night Massacre," the recently confirmed Associate Attorney General Rachel Brand may soon inherit a key responsibility in the Justice Department chain of command. If Rosenstein recuses himself or is fired, it could be Brand's role to supervise the Mueller investigation and execute any lawful orders issued by her boss, the President of the United States. The President's Twitter trail suggests that firing Mr. Mueller could soon be on Ms. Brand's busy agenda.

Note: An earlier version of this article incorrectly described the chain of command within the Justice Department. The recently confirmed associate attorney general would take over if the deputy, Rod Rosenstein, is unable to serve.