ST. PAUL, Minn. — A Democratic legislator from Minnesota swiftly apologized Tuesday for a tweet he sent that referred to Supreme Court Justice Clarence Thomas as “Uncle Thomas” following a major ruling on the nation’s landmark voting rights law.

Thomas, who is black, was part of a 5-4 majority that invalidated part of the Voting Rights Act meant to deter racial discrimination in elections. The ruling makes it tougher for federal officials to prevent states and localities, primarily in the South, from adopting policies that add barriers to voting.

In response, state Rep. Ryan Winkler tweeted: “#SCOTUS VRA majority is four accomplices to race discrimination and one Uncle Thomas. Marriage decision may blur Court’s backsliding.”...