Why Senate polling is always worse in summer

For four cycles in a row, the polls have overestimated Democrats. Here’s what history shows us happens next.

Current polling looks good for Democrats in many red states they’d need to flip control of the chamber. That includes a recent Fox News poll that puts Sherrod Brown up 8 in the Ohio Senate race or a spate of polling that has James Talarico leading in Texas and Mary Peltola leading in Alaska.

Democrats look like they’ve had a good chance in red states before. A July 30 - August 3, 2020 Quinnipiac poll showing a tied race for South Carolina Senate race between the late Lindsey Graham and Jaime Harrison kicked off a trend of neck-and-neck polls that continued almost until the very end. Graham would eventually win by 10. In 2022, Democrat Tim Ryan led in the vast majority of polls conducted in Ohio over the summer until being bested by JD Vance by 6 points. The same pattern repeated itself in Ohio in 2024 when Brown led in all but two pre-October polls in his race against Bernie Moreno.

In the last few election cycles, we’ve seen a pattern of Democrats looking competitive in red state Senate contests during the summer only to fade by election day. I wanted to test if this was a consistent pattern, not just the selective memory of a Republican.

3,312 polls over the last four election cycles in the old FiveThirtyEight archive confirmed the pattern. In each of those four cycles, the Democratic candidate’s margin was overstated. The average polling error across all polls was D+3.7, and 73% of polls released overstated the Democrat relative to the election result. Again, that’s across more than 3,000 polls, so that’s hardly just statistical noise.

And the pattern is most pronounced during the summer, right where we are now. Polling error peaks at 5.9 points too Democratic sometime in late July, and doesn’t start coming down until late August, before settling into an average error of D+2.6 on election day.

But this isn’t all.

There’s a dramatic difference between midterm and presidential cycles. In the Trump era, Senate polls in presidential years have been biased towards Democrats, and they remained biased until the very end. Polls in midterm years ended up pretty accurate by election day, but were very biased in the summer.

Here’s what this analysis of 3,000-plus polls with 11 charts breaking down this phenomenon reveals — from midterms vs. presidential cycles, which states and with which demographics the errors are worst in, and what you as a consumer of polls need to know.