Rep. Joe Kennedy III, D-Mass., announced Saturday that he will mount a primary challenge to Sen. Ed Markey, D-Mass., next year. The 38-year-old Kennedy -- the grandson of Robert F. Kennedy – formally announced his candidacy at a community center in Boston after weeks of rumors that he was mulling entering the race. [Snip] The challenge to Markey, 73, is likely to leave local Democrats, including 2020 presidential hopeful and fellow Sen. Elizabeth Warren, D-Mass., with a difficult choice of who to back. Rep. Alexandria Ocasio-Cortez, D-N.Y., has already endorsed Markey, with whom she has worked closely on the Green...