President Obama’s civil-rights legacy looked on track, not long ago, to include a major push against America’s deeply entrenched housing segregation. In 2015, his administration rolled out a rule requiring local communities to assess their own patterns of racial and income segregation and make genuine plans to address them.

The move followed years of debate and came as segregated cities like Baltimore and Chicago faced renewed bouts of racial unrest. The federal government, advocates hoped, was finally trying to repair a long-unkept promise of the 1968 Fair Housing Act.

Now that rule is likely to be undermined — and possibly erased — by a Department of Housing and Urban Development headed by Ben Carson. On Monday, president-elect Donald J. Trump officially offered the cabinet post to Mr. Carson, a neurosurgeon and a former presidential candidate, who grew up poor in Detroit but has no experience in housing policy.

While we know little about what Mr. Carson would do at the agency, he has played down the role of government in his own up-from-urban-poverty story. (“If you don’t succeed,” his mother taught him, according to his autobiography “Gifted Hands,” “you have only yourself to blame.”) And he has specifically criticized the Obama housing rule.