Fred Harris, a former U.S. senator from Oklahoma, presidential hopeful and populist who championed Democratic Party reforms in the turbulent 1960s, died Saturday. He was 94.

After eight years in the Senate as a moderate Democrat, he took a leftward turn toward “new populism” in a failed shot at the presidency in 1976.

Mr. Harris, a liberal populist, was once considered one of the bright young faces of the Democratic Party. He twice ran for president and chaired the Democratic National Committee.