There are three main reasons that primary surveys are less accurate, and more difficult to conduct, than general election surveys. First, voters do not pay much attention to primary elections, so they are more likely to be influenced by election reporting, new information, and notable events–big events and perceptions of momentum can swing voters from one candidate to another. Swings in polling margins are one result of this inattention.
Second, uncertainty about candidate preferences happens in closed primary states like Pennsylvania because people are voting for candidates who represent their chosen party. Primary candidates’ issue positions are much more similar than they are when candidates from opposing parties face each other in a general election. This increases voter uncertainty about who they will vote for and also makes their preferences less stable and less certain.
Third, voters in primaries are less certain about whether they will actually vote. A vital task in getting an accurate picture of candidate support is determining who will vote, but primary elections make it harder to figure this out because far fewer people vote in primary than general elections.
Not surprisingly, pollsters working in a primary election have a harder time telling who will or will not vote. And this point is essential—to get accurate results pollsters must talk to those who will vote and identify those who will not.
Voter uncertainty is of course not the only thing that matters—technical choices in how voters are sampled and interviewed, how questions are designed, how “likely voters” are estimated, and how the results are interpreted also contribute to some of the differences we see in polling.
But polling can only produce accurate results when the voters themselves are sure about their intentions. The truth is that in primary elections voters take much longer to make up their minds and wait until much closer to Election Day to do so. Primary election polling simply reflects voters’ uncertainty.