Truman won this way in 1948, it's never over until every vote has been counted.
This is of course true, and is a good reminder why no side should ever be overconfident. But 1948 is not really comparable. Polling then was much scarcer and not as representative as it is today. But even with that, Truman's deficit in the polls was much less than Trump's is today.
Trump is in the position of a football team that's down by two touchdowns with 2 minutes to play and the ball on their own 20. Is it possible to come back from such a deficit? Sure, it's happened. That's why the side that's down shouldn't give up hope, and the side that's ahead shouldn't assume it's over until the final gun sounds. But is such a comeback likely? No, not at all.
