When Senator Elizabeth Warren, of Massachusetts, took up the fight against Donald Trump in the last few weeks, it felt a little like that moment in an action movie when the disaffected friend who’d walked out turns up late in the third act, mid-fray, with fresh ammunition and a wink and a grin for her old gang. With Hillary still pinned down in the primaries, unable to shake off the dogged Bernie Sanders campaign, Warren started a Twitter war with Donald Trump.
