Former president Barack Obama hasn’t endorsed Kamala Harris’ presidential bid because he doesn’t think she can beat Donald Trump, according to a source close to the Biden family. Following Joe Biden’s shock resignation from the race on Sunday, and his immediate endorsement of the vice president, most of the Democratic elite have been quick to rally behind Harris — but Obama is a notable exception. “Obama’s very upset because he knows she can’t win,” the Biden family source told The Post. “Obama knows she’s just incompetent — the border czar who never visited the border, saying that all migrants should...