Both Kerry and Romney's bases were absolutely convinced that they were going to win prior to Election Day and both used flawed methodology to back their arguments, I remember the weekend before the 04 election a popular theory emerging on left-wing blogs that liberals were more likely to only have cell phones which weren't polled and as a result the polls were skewed against Kerry.
At least one Bush supporter was convinced Bush was going to lose.
https://uselectionatlas.org/FORUM/index.php?topic=12005.0
