As the Democratic presidential contenders begin a four-month sprint to the Iowa caucuses, the front-runner, Joe Biden, is in trouble. Biden surged to the lead even before he announced, buoyed by name recognition, experience and his service as vice president to Barack Obama. He was anointed the “most electable” of Democrats. But, now, his early lead in polls is fading, with Sen. Elizabeth Warren (D-Mass.) even inching ahead of him in a recent Iowa poll. This marks only the beginning of Biden’s fall: In reality, as findings from a new book by veteran Democratic pollster and strategist Stanley Greenberg suggest, among the lead contenders Biden might well be the weakest potential opponent to President Trump.