[lancer]

Pollsters learned a lot from that 1948 fiasco, like not polling from phone books when most of middle America didn't have phones...

[onyx]

Right.
The polling was too slow to keep abreast of the changing mood of the voters.

[Brandon]

1936 was the election you're thinking of -- Literary Digest did a telephone poll, and since mostly wealthy people had phones, and they were mostly Republicans, they got a bad result.

What happened in 1948 was that pollsters didn't realize how quickly public opinion could shift. They saw Dewey with what they thought was an insurmountable lead, and stopped polling about a month or six weeks before the election -- why waste money when it's a done deal, right? Truman proved them wrong.