When comparing the findings from The Field Poll’s final pre-election survey of likely voters (n-966) to the Edison Media Research exit poll in California, the biggest differences relate to the turnout and preferences of frequent church-goers and Catholics. The Field Poll, completed one week before the election, had Catholics voting at about their registered voter population size (24% of the electorate) with voting preferences similar to those of the overall electorate, with 44% on the Yes side. However the network exit poll shows that they accounted for 30% of the CA electorate and had 64% of them voting Yes. Regular churchgoers showed a similar movement toward the Yes side. The pre-election Field Poll showed 72% of these voters voting Yes, while the exit poll showed that 84% of them voted Yes.
The same kind of phenomenon occurred when the first same-sex marriage ban was voted in California in the March 2000 election (Prop. 22), although because of the size of its victory( 61% Yes vs. 39% No) it didn’t matter much back then. In that year The Field Poll’s final pre-election poll, also completed about one week prior to the election, had 50% of Catholics on the Yes side, and accounting for 24% of the vote. Yet, the network exit poll conducted that year by Voter News Service showed them to account for 26% of the electorate with 62% voting Yes.
My take is that polling on issues like same-sex marriage that have a direct bearing on religious doctrine can be affected in a big way in the final weekend by last minute appeals by the clergy and religious organizations.