The memo contrasted Ms Selzer’s findings with the Emerson College poll, arguing that the second survey was more similar to exit polls in 2020.
Pundits suggested that if Ms Selzer’s poll was correct, it could be explained by a boost in support for Ms Harris among women, because of a six-week abortion ban which took effect in Iowa in July.
Ms Harris has made her opposition to abortion bans a major feature of her campaign, and has promised to reinstate federal protections for abortion access if she is elected on Tuesday.
Whichever candidate wins Iowa will collect six Electoral College votes. A total of 270 are needed to win the election, of 538 available from all states.
Both candidates have paid little attention to Iowa during the campaign, preferring to hold rallies in the battleground states of Wisconsin, Michigan, Pennsylvania, North Carolina, Georgia, Arizona and Nevada.
Nationally, Ms Harris is ahead of Trump by one point, according to an average of polls collated by FiveThirtyEight.