Former President Donald Trump’s new running mate, Sen. J.D. Vance (R-OH), is emblematic of a deeper intellectual rot at the heart of the newer generation of Republican thinkers, wrote MSNBC’s Chris Hayes in a scathing thread on X Tuesday evening. Vance, a former venture capitalist who became famous for his “Hillbilly Elegy” memoir of growing up in western Ohio, started out as a never-Trump conservative who proclaimed Trump could become “America’s Hitler” — but rapidly changed his tune when it came time to run for Senate in Ohio, going full-blown MAGA and now proclaiming he would help Trump ove…