Has any modern presidential challenger held a polling lead in October and not gone on to win the November election? Has any modern President lost a lead in October and gone on to win re-election? I honestly don’t know. I doubt it.
UPDATE: Based on my cursory reading of Gallup’s historical data, no incumbent President that lost a polling lead in October has gone on to win re-election before or after the infamous polling disaster of Truman vs. Dewey in 1948. Any challenger that had a lead in October has gone on to win the election.