Walter M. Shaub Jr., former director of the US Office of Government Ethics, is a CNN contributor and senior adviser for Citizens for Ethics and Responsibility in Washington, a nonpartisan organization that advocates for strong ethics oversight and accountability for government officials. The opinions expressed in this commentary are his own; view more opinion at CNN.

(CNN) Richard Nixon must have wished for friends in Congress like the ones Donald Trump has. Nixon resigned in 1974, when any remaining loyalists in Congress found themselves outnumbered. Not so this time. The Senate is about to ratify President Trump's slow-motion sequel to the Saturday Night Massacre.

In the original Saturday Night Massacre , the attorney general and his deputy refused to carry out Nixon's order to fire the special prosecutor investigating the Watergate scandal. Moving down the line after they resigned, Nixon found that Solicitor General Robert Bork was willing to do the deed.

The resulting public outcry forced Nixon to agree that he would let Bork appoint another special counsel and would not fire this one without the concurrence of a group of eight members of Congress. This agreement wasn't enough to save Nixon's presidency, though, and, among other things, his subsequent forced resignation established the principle that a president may not select his investigator.

Now with the Senate's likely confirmation of William Barr as attorney general, Trump may succeed in destroying this principle. Barr's nomination is before the Senate only because Trump fired Attorney General Jeff Sessions for refusing to stop special counsel Robert Mueller's investigation. (Sessions technically resigned, but a "resignation" requested by the president is Washington-speak for "fired."

Trump, who had railed against Sessions since Mueller's appointment, later tweeted that "Jeff Sessions should be ashamed of himself for allowing this total HOAX to get started in the first place!"

Sessions was not the first casualty of Trump's attacks on investigators. Trump fired former Federal Bureau of Investigation Director James Comey for investigating him. He also successfully advocated for the firing of career FBI officials Andrew McCabe and Peter Strzok.

Whatever level of culpability McCabe and Strzok may have had in violating Justice Department rules or procedures, Trump's interference tainted the personnel actions against them. Presidents are expected to stay out of career-level personnel actions to protect certain functions of government from political influence. Indeed, Congress investigated the firing of career officials in the White House travel office under President Bill Clinton. Rather than doing the same during the Trump administration, the congressional majority launched its own attacks on career officials.

Barr is infinitely more qualified than acting attorney general Matthew Whitaker, but Barr and Whitaker have something in common: They both auditioned for the job by making sure Trump knew they opposed the special counsel investigation. Whitaker made his views known in television appearances and op-eds , and Barr sent the Justice Department an unsolicited 20-page memorandum challenging the scope of the investigation.

Barr has also displayed his partisanship in the media. In an October 2016 Washington Post opinion piece headlined, "James Comey did the right thing," Barr defended Comey for releasing information about an investigation of presidential candidate Hillary Clinton shortly before the election. Trump later fired Comey. Then, on May 11, 2017, Trump admitted on television that the firing was motivated by the investigation of his campaign. The next day, Barr raced to Trump's defense with a new opinion piece condemning Comey for having released information about the Clinton investigation.

Barr tried to explain away the incongruity in his writings by claiming his October 2016 piece was based on a mistaken belief that Attorney General Loretta Lynch had authorized Comey to act independently. But Barr's October 2016 piece never mentions Lynch and, instead, asserts that Comey had a duty to refute statements made by the Clinton camp. What changed between October 2016 and May 2017 was that Barr learned Comey was also investigating Trump.

Barr's shifting opinion of Comey is consistent only in its unquestioning loyalty to Trump. Loyalty, after all, is the quality Trump prizes most in his appointees. This month, Whitaker raised eyebrows with his praise of Trump for working over the holidays. In contrast, Sessions' decision to recuse himself from oversight of the special counsel investigation led Trump to lament, "I don't have an attorney general. It's very sad." Indications are that Trump won't have the same problem with Barr, who has a reliable track record.

Barr supported President George H.W. Bush's granting clemency to former officials caught up in the Iran-Contra scandal. NPR reports that Barr later said, "I favored the broadest pardon authority" and "There were some people just arguing just for (Caspar) Weinberger. I said, 'No -- in for a penny, in for a pound.'"

Barr has said he will not end the special counsel investigation, but that is no comfort. Barr is a savvy Washington operator who would know how to use his authority to influence the investigation in subtle ways. He may narrow its scope, influence key investigative or prosecutorial decisions, or edit the final report. If Barr has the principles to put his country first, he will recuse himself entirely from the investigation.

But the problem is bigger than Barr. Confirming any Trump nominee for the attorney general position, without requiring the nominee to commit to recusing from the special counsel investigation, would put an end to the principle that presidents may not choose their investigators. The Senate majority put this principle on life support when it confirmed an FBI director after Comey's firing. If it confirms a replacement for Sessions without demanding recusal, the Republican majority will pull the plug on the patient.

Maybe Mueller and the US attorneys will yet outwit the conspirators against them, but Barr's confirmation without a commitment of recusal would change the presidency. It would set a dangerous new precedent that presidents are free to fire law enforcement officials for investigating them, and to choose their replacements.

To protect the rule of law, the Senate must demand, as a condition of confirmation, that Barr agree to recuse himself from the special counsel investigation.