On Friday, Noam Scheiber took an exclusive look at the Romney campaign's internal polling and talked to Romney's chief pollster, Neil Newhouse, about why Romney thought he was going to win. Ed Kilgore zeroes in on the campaign's embrace of its own propaganda:

Aside from succumbing to the “enthusiasm” myth, it seems Romney and his staff also bought into the “momentum” myth: the powerfully seductive belief that sometimes-random polling gains represent an irresistible trend. Newhouse saw a significant jump in Mitt’s numbers in battleground states the Sunday before Election Day, and like his colleagues thought this was “momentum” that would probably continue right through November 6.

Harry Enten adds:

[T]he Romney campaign somehow came away with the belief that their candidate would win, even as their own optimistic surveys showed him only getting to 267 electoral votes. It came from their belief that they had momentum in Ohio, which, as discussed, was based on the faulty notion that you see big dips and dives in polls outside major campaign events. As I noted in the lead-up to the election, Romney didn't lead in a single public Ohio poll in the final weeks. Historically, this has always been a death knell. Romney didn't lead in a single internal Ohio survey, either.

Among the lessons Nate Silver draws:

A pollster working within a campaign may face a variety of perverse incentives that compete with his ability to produce the most accurate possible results to his candidate. He may worry about harming the morale of the candidate or the campaign if he delivers bad news. Or he may be worried that the campaign will no longer be interested in his services if the candidate feels the race is hopeless.