The Justice Department building in Washington, D.c. (Yuri Gripas/Reuters)

IN HIS 1991 confirmation hearings, C-SPAN noted on Friday, former attorney general William P. Barr declared that “nothing could be more destructive of our system of government, of the rule of law, or Department of Justice as an institution, than any toleration of political interference with the enforcement of the law.” Now that President Trump has nominated Mr. Barr to return to the Justice Department’s helm, the Senate and public must ask whether he will live by those words almost three decades later.

Mr. Barr’s nomination comes as a relief. Rather than tapping a confirmed Trump sycophant such as former New York mayor Rudolph W. Giuliani (R), the president picked a Justice Department veteran of undoubted legal professionalism.

Yet given Mr. Trump’s degradation of the executive branch; his past statements indicating he expects the attorney general to protect him personally; and the fact that federal prosecutors on Friday connected the president to a felony campaign finance violation by his former personal lawyer, special scrutiny of Mr. Barr’s commitment to Justice Department independence is essential.

Under President George H.W. Bush, Mr. Barr operated with admirable autonomy. In 2001, he revealed that he had insisted on naming his own deputy attorney general rather than allow the White House to install someone based on political considerations. He noted that his Justice Department defended laws in court whether or not he thought they were proper (in contrast to the Trump administration’s refusal to defend the Affordable Care Act). Mr. Barr oversaw federal charges against the officers involved in the Rodney King beating, and he has boasted of the warm relations he maintained with members of both parties in Congress.

On the other hand, though he appointed an independent counsel during his time at the Justice Department, he has been a fierce critic of such special investigators. Senators should probe how involved he was in undercutting an independent counsel investigation into the Reagan-era Iran-contra scandal. His views do not prove he would improperly meddle with special counsel Robert S. Mueller III’s Russia investigation, but senators should explore them.

They should also insist that he reveal what Mr. Trump asked him before nominating him. In 1991, Mr. Barr told senators that the “only charge” Mr. Bush gave him “was to have a Department of Justice that was run with professionalism and integrity.” What charge did Mr. Trump give him? And does he still believe that repeatedly debunked Hillary Clinton “scandals” deserve more Justice Department scrutiny than questions about Trump campaign collusion with Russia?

Senators also should not lose sight of the fact that Mr. Barr would run a sprawling department overseeing important federal policy. What would be his priorities in enforcing federal voting rights law? How would he approach immigration issues? Would he impose the outmoded get-tough-on-crime ethos of the 1990s Justice Department? Would he, like recently fired attorney general Jeff Sessions, take pressure off police departments to reform?

Mr. Barr said in 1991 that he wanted his legacy to be that “I upheld the law and that I left the Department of Justice a more effective, stronger and a better institution.” If that is once again his aim, his nomination is welcome.