Sen. Bernard Sanders is leading a bloc of the U.S. Senate’s most liberal members to push Democratic Party leaders to require all the party’s candidates to reject billionaire‑ and corporate‑backed super PAC spending in their primary campaigns. In a letter to Senate Minority Leader Charles E. Schumer and Democratic National Committee Chair Ken Martin, the senators praised a recent DNC resolution condemning dark money but said it doesn’t go far enough. “We cannot allow unlimited outside spending