 The uncomfortable occurrences were not predictable, but seemed more likely when he was in a large crowd or tired after a particularly bruising schedule. In the 23 days leading up to the debate against former President Donald Trump, Mr. Biden jetted across the Atlantic twice for meetings with foreign leaders and then flew from Italy to California for a splashy fundraiser, maintaining a grueling pace that exhausted even much younger aides. So their excuse now is that Joe Biden got very tired from having been in Europe. There is only one problem. He arrived back in the United States 12 days before the debate.