Are the Polls Still Missing ‘Hidden’ Republicans? Here’s What We’re Doing to Find Out.

That said, an initial glance at the topline findings may be sobering for anyone who hoped that $25 and higher response rates would break through to reach the coveted “hidden” Trump vote. While there were important differences between the high- and low-incentive surveys — including some that hold promise for improving Times/Siena surveys and others going forward — there was not necessarily obvious evidence of a breakthrough to a vastly different pool of respondents.

While the mail survey showed the Republican incumbent, Ron Johnson, and the Democrat Mandela Barnes tied among registered voters — a tally that’s similar to the results of other polls — the raw, unadjusted respondents to the mail and telephone surveys both leaned Democratic by a considerable margin, including a six-point margin in the race for U.S. Senate in the high-incentive mail survey.

(It’s important to note that these poll results are “old” — both the mail-in surveys and the Times/Siena polls were conducted over a period of weeks, beginning in September. It would be a mistake to assume these results are representative of voters today.)

The results by demographic group were uncannily similar as well. White working-class voters — both before and after weighting — had nearly identical partisan preferences in the two surveys. Registered voters in Wisconsin’s Third District — the Obama-to-Trump district in the state — backed Mr. Barnes in both surveys.

The relatively small differences between the high-incentive mail survey and other polls could be framed as a good or bad thing for surveys — and it raises the polling stakes for today’s election.

On the one hand, the small to modest differences suggest that the Times/Siena poll and other low-incentive surveys remain competitive with a high-incentive survey with vastly higher response rates. On the other, it might be interpreted to mean that the “hidden Trump” vote remains out of reach — that $25 can’t reach a far more representative sample. (Of course, we won’t know which of these is true until the election is over.)

The Times/Ipsos mail study offered relatively little evidence to support some of the most popular theories about nonresponse bias, like social trust.