THE JUSTICE DEPARTMENT owes its very existence to the struggle for greater equality in the United States, especially greater racial equality. Congress created it in 1870 largely for the purpose of enforcing Reconstruction-era laws designed to protect newly freed slaves in the South from the Ku Klux Klan and others who would deny them the franchise and civil rights. During the 1960s, the Justice Department stood at the forefront of federal efforts to make good on the promises to African Americans that it had failed to fulfill in the post-Civil War era.

Now President-elect Donald Trump has chosen Sen. Jeff Sessions (R-Ala.) to head this vital agency, whose modern writ still includes the protection of civil rights and has expanded well beyond it to encompass prosecuting a vast array of federal crimes, exercising oversight when local police departments stand accused of brutality, supervising immigration judges and fighting terrorism consistent with civil liberties. The attorney general “sets the priorities for law enforcement throughout our country. And all of this must be done with objectivity and impartiality,” as Sen. Dianne Feinstein (D-Calif.) aptly noted Friday.

None can question either Mr. Trump’s right to pick the 69-year-old Mr. Sessions, or the latter’s credentials, in terms of experience, as a former U.S. attorney and legislator. What many do question, however, is Mr. Sessions’s commitment to the full historic range of Justice Department missions. They are right to do so, and not simply because he made offensive racial remarks in conversations more than three decades ago, comments which cost him confirmation to a federal judgeship during the Reagan administration. Even more relevant is Mr. Sessions’s Senate record over the past two decades: hostility to immigration reform bills, unrelenting skepticism toward vigorous federal enforcement of the Voting Rights Act and opposition to moderate gun-control measures such as limits on high-capacity magazines for firearms. Mr. Sessions voted for a constitutional amendment to ban same-sex marriage in 2006 and against the 2010 repeal of “don’t ask, don’t tell,” the policy that banned gay men and lesbians from serving openly in the military.

A backer of the Trump candidacy long before most of his colleagues among GOP elected officials, Mr. Sessions supported the billionaire unstintingly through his various campaign outrages. And now he has been rewarded. As we said, his nomination is a legitimate exercise of the power Mr. Trump won on Nov. 8. Equally legitimate, and utterly necessary, will be a thorough, penetrating Senate confirmation process, in which Mr. Sessions is vigorously challenged to explain how he intends to provide the objective, impartial justice of which Ms. Feinstein spoke, in the eyes of all citizens. The record suggests Mr. Sessions carries a heavy burden of persuasion, and carry it he must, lest millions of Americans lose faith in the nation’s highest law enforcement official and the proud, powerful department he would head.