Former Vice President Joseph R. Biden on Monday said that he would consider choosing a Republican for a running mate if he’s the 2020 Democratic presidential nominee, though he said someone who would fit the proper bill didn’t immediately come to mind. “The answer is I would, but I can’t think of one now,” Mr. Biden said in response to a questioner at a campaign stop in New Hampshire. “No, I’m serious. … There’s some really decent Republicans that are out there still, but here’s the problem right now with the well-known ones: they’ve got to step up. You know...