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) Evidence is rapidly accumulating suggesting that the next volley in President Donald Trump's war on special counsel Robert Mueller will be the firing or forced resignation of Attorney General Jeff Sessions. If Sessions is out, he will presumably be replaced by an amenable attorney general who will likely terminate Deputy Attorney General Rod Rosenstein as well.

It is as clear as the view from the Trump Tower where this road leads. Trump is bent on ousting the two biggest obstacles at the Justice Department to his real objective: getting rid of Mueller. While inevitably such a step would draw immediate comparisons to Richard Nixon's " Saturday Night Massacre ," Trump also has far more political cover to take a clear shot at the special counsel he reportedly so reviles.

In his latest presidential tweet on the matter, Trump assailed Sessions for a "very weak position" on Hillary Clinton's "crimes" -- less than a full day after he dismissed his attorney general as "beleaguered." And beleaguered he should be, since the President who selected him is his "beleaguerer in chief," given the President's comments to The New York Times that Sessions should never have recused himself from the Russia investigation and that had the President known of Sessions' intentions in this regard, he never would have appointed the senator from Alabama to his prestigious and powerful position in the first place.

Interestingly, however, Rudy Giuliani -- a fervent supporter of the President and past contender for the post Sessions now holds -- told CNN he thought Sessions had made the right decision recusing himself from the Russia investigation. Giuliani made no comment in this interview regarding the propriety of Sessions' failure to mention to the President (before his appointment) that he planned (before his appointment) to recuse himself, which is Trump's real beef.

Trump's recent trolling of Sessions followed previous presidential criticism of Rosenstein , the man left to supervise the Russia investigation after Sessions' recusal over his own potential conflict of interest.

The President took aim at Rosenstein on Twitter last month, describing the Justice Department's investigation as "phony" and "sad" and also asserting that "I am being investigated for the firing of the FBI Director by the man who told me to fire the FBI Director." The reference here, of course, is to the now-infamous " Rosenstein memo " that listed numerous criticisms of FBI Director James Comey shortly before the President summarily terminated him.

Mueller's investigation into Russian election tampering and possible collusion by the Trump campaign will necessarily require an examination of whether the firing of Comey was possibly an illegal obstruction of justice designed to impede the investigation.

One of the key witnesses is obviously Rosenstein, who will undoubtedly have to testify under oath about his memo and meetings with the President. If the President was following the perfectly legal guidance of Rosenstein (a position that, incidentally, Trump appeared to contradict in an interview with NBC's Lester Holt), he could not have been obstructing justice, which requires a malicious illegal purpose.

Here's the bottom line: Sessions had initially undisclosed contact with the Russians, and Rosenstein may well be the key witness on the issue of why the President really fired Comey. As such, both have potential conflicts of interest in how this all plays out.

Given the potential conflict and the President's negative attitude toward him, Rosenstein's position is even more precarious than that of Sessions. At least Sessions was an early and important supporter of the President. Rosenstein, on the other hand, came into the No. 2 spot in the Justice Department as an outsider with no real Trump campaign or administration connections. He holds only an integrity card, which is not exactly the coin of the realm in the Trump camp.

The forced (or encouraged) departures of Sessions and Rosenstein will enable the President to fill the top two spots at Justice with loyal supporters who will become Mueller's new bosses and will demand an immediate report on the status and progress of the Mueller probe.

When you factor in the previous work done by the FBI before Mueller's appointment, this investigation has been going on for more than a year without any publicly disclosed evidence of the Trump campaign's active collusion with the Russians. Yes, Donald Trump Jr. expressed a desire to get dirt on Clinton from the Russians, but wishing doesn't make it true. No smoking gun has yet to be dredged from the detritus of the successive probes. Whoever the new attorney general turns out to be, it won't be hard for him to assert that the probe is a waste of money staffed by contributors to the Democratic Party in Mueller's office.

These are the talking points Trump's supporters will likely use to defend what will surely be compared to the "Saturday Night Massacre."

Unlike Nixon, though, with a Republican Congress in place, Trump may at least for the time being have the political ability to survive the ouster of Sessions, Rosenstein and even Mueller. If the 2018 congressional elections result in a Democratic majority in Congress, Trump will lose his ability to survive a Trump "Saturday Night Massacre," so he must act swiftly to put "Mooch"-like supporters in the top Justice Department spots. ("The Mooch" is the nickname of the new White House communications director, Anthony Scaramucci , who once called Trump a "hack" but now says those comments are "one of the biggest mistakes" he's ever made.)

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Giuliani's name is being mentioned, but he would likely encounter too much public opposition to be confirmed. Chris Christie has similar problems, so expect a new name to emerge in the coming days.

The threat that Trump apparently fears most is grand jury subpoenas subjecting his business empire and his tax returns to careful Mueller scrutiny. The Mueller investigation will soon be approaching the "red line" bordering the territory that the President so desperately wishes to shield from public and prosecutorial scrutiny. Sessions, Rosenstein and Mueller are likely to be on their way out in the near future in service of that objective.