Four years ago, Donald Trump prepared to debate his general-election opponent for the first time. Down in the polls to an experienced, traditional pol, he had been reduced to spreading weird rumors and casting doubt on the legitimacy of the vote, even as questions swirled about his personal finances. Now Trump is the incumbent president, and the conditions could not be more different as he prepares for his first debate with Democratic nominee Joe Biden on Tuesday: a nation wracked by disease, disorder and disasters; an election neither candidate is treating like a foregone conclusion.