Sen. JD Vance speaking to supporters in Grand Rapids, Michigan, earlier this month.Alex Wroblewski/The Washington Post via Getty ImagesIn the days since he became Trump's running mate, JD Vance's vulnerabilities have become clear.Until now, Vance was mostly unknown nationwide. That makes it easier for opponents to define him.He's also long espoused ideas that are generally unpopular with the public.Nearly two weeks after former President Donald Trump selected Sen.