So you can look at Gallup's own analysis of their polls for this. They mostly blame the polls ending weeks before the election, which seems like a pretty big oversight. The election was fairly close, there were two strong third-party candidates, and Truman had been campaigning hard in those last months. So the polls could have been perfectly accurate (although this may or may not be likely) and their failure to poll people when it mattered would have still made the difference. Here's the article Gallup put out: http://www.gallup.com/poll/7444/gallup-brain-strom-thurmond-1948-election.aspx