Sen. J.D. Vance just made a campaign fumble — dropping a ball the Democratic presidential nominee should pick up and run with, according to a new political analysis. Former President Donald Trump’s running mate’s attempt to fill in the gaps of his “concept of a plan” to replace the Affordable Care Act included an unpopular policy his political foes should jump to disavow, MSNBC opinion writer James Downie wrote Monday. “Vance confirmed that Trump wants to revive one of the worst aspects of American health care before the Affordable Care Act,” Downie wrote, “bringing health care into the electi…