WEDNESDAY MARKS the 150th anniversary of the Department of Justice, which is both auspicious and, to many Americans, possibly surprising. Central as it is to American government, this Cabinet-level agency did not exist at the founding. There was little in the way of federal law, civil or criminal, until the Civil War. In the aftermath of that conflict, however, Congress established the department. The reformist purpose was to get rid of the expensive private attorneys — often party hacks — whom the federal government had previously hired to represent it, and to replace them with full-time professionals supervised by the attorney general. Simultaneously, Congress gave the department a new responsibility: to enforce the first-ever federal statutes designed to protect the civil rights of African Americans.

Awareness of this history only compounds our dismay at the department’s course under the Trump administration and the most recent attorney general, William P. Barr. Instead of impartiality and professionalism, there have been presidential interference with policy and personnel decisions and unjustified leniency for political favorites. Instead of an aggressive civil rights posture, there have been reluctance to investigate systemic violations by police departments, backing for lawsuits against affirmative action in college admissions and, from Mr. Barr himself, support for a police sweep of nonviolent protesters in Lafayette Square to make room for President Trump to stage a photo op.

Some partisanship, real and perceived, is inevitable given that the ultimate power over federal law enforcement resides with politicians in Congress and the White House. In that sense, creating the department in 1870 was, at best, a partial fix for a dilemma inherent to the Constitution. From the beginning, and increasingly since the mushrooming of federal law enforcement authority in the 20th century, critics have accused the Justice Department of undermining the rule of law rather than upholding it, as anyone familiar with the careers of A. Mitchell Palmer or John N. Mitchell — or even Robert F. Kennedy — knows. The troubling covert surveillance warrants against Trump supporter Carter Page belong on that list, too.

One way to reduce the risk of politicized or otherwise abusive federal law enforcement may be to reduce the sheer scope of federal criminal law, which has become overgrown and duplicative. As part of criminal justice reform, Congress should trim the excess, consistent both with the Constitution’s assignment of primary responsibility for crime control to the states, and with the efficiency-promoting aims of the department’s founders.

Ultimately, though, a large role for the Justice Department must remain — the legitimacy of which will depend on the ethos of its leaders, up to and including the president. Despite its imperfections, this is an indispensable institution; its career staff represents a vital national repository of public-spirited talent. These officials embody the legacy of reform and heightened independence in the post-Watergate years — a period that provides a hopeful precedent for today.

More than new policies or procedures, the Justice Department needs rededication to its founding spirit — professionalism and impartiality — and renewed emphasis on its oldest mission: civil rights. The Trump administration has shown itself incapable of that rededication. The next president and attorney general will have to do the job, urgently.

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