Why did this poll get so much attention?
The final Des Moines Register/Mediacom Iowa Poll from Selzer & Company put Harris ahead 47–44. In a closely watched presidential campaign, an unexpected Democratic lead in Iowa was a conspicuous point estimate.
It was still one poll, with its own sample and likely-voter screen. A three-point lead was not a high-confidence claim that Harris would win the state; the reported candidate-share sampling margin was 3.4 points, and uncertainty in a candidate lead is larger than uncertainty in either share alone.
What did the final vote show?
Iowa’s certified canvass recorded 927,019 Trump votes and 707,278 Harris votes. Using 1,663,506 valid presidential votes gives Trump a 13.2-point margin. Selzer’s D+3 margin missed that result by 16.2 points.
That is an error in the difference between two candidate shares. It does not mean both candidates’ vote shares were each wrong by 16.2 points. Undervotes and overvotes are not candidate votes and are excluded from this denominator.
Did every Iowa poll show the same thing?
No. Emerson’s November 1–2 poll put Trump at 53 and Harris at 43, a ten-point Republican lead. Against the same outcome, its margin miss was about 3.2 points.
The comparison illustrates substantial disagreement between polls. Their interviewing periods, samples and methods differ, so it is not a controlled test establishing why one method worked better. The original unallocated Emerson 53–43 result is used rather than its separate estimate reallocating undecided voters.
Why did Selzer miss by so much?
Selzer said in a December 13, 2024 Iowa PBS interview that she had not established why the poll missed. She discussed the likely-voter screen, response patterns and possible late turnout changes as possibilities; none was demonstrated as the explanation in that interview.
A large residual tells us that the estimate was far from the outcome. It does not, on its own, tell us which people failed to respond, which voters were screened incorrectly, how preferences changed afterward, or how much each mechanism contributed.
What can we conclude—and what remains open?
The selected poll’s margin was a major miss. The existence of another poll much closer to the result prevents treating the Selzer result as the error of every Iowa poll.
The explanation remains an empirical question. Read the firsthand post-election interview and the original methods before adopting a single-cause theory. Neither the size of the error nor the pollster’s reputation substitutes for that evidence.
What to carry forward
One prominent outlier can dominate the election story. Evaluate it against the result and other polls without pretending that its cause has been established.