Centrist Chris Pappas faces progressive primary challenger Karishma Manzur while Republicans eye chance to flip seatNew Hampshire voters head to the polls Tuesday for primaries that will set up a closely watched Senate contest – a vulnerable open seat Democrats must defend if they are to have any shot at reclaiming the upper chamber.The race opened when Jeanne Shaheen, 79, the state’s three‑term incumbent Democratic senator, announced her retirement. Congressman Chris Pappas, who represent