An investigation of U.S. Senate polling shows 73% of surveys overstate the levels of support for Democrats. Patrick Ruffini, the co-founder of Echelon Insights, published the results of the deep dive into polling on Tuesday.

The investigation looked at over 3,000 polls over the last four election cycles, and the greatest disparity occurs over the summer months.

“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,” Ruffini writes on his Substack.

“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,” he says.

The pattern, according to Ruffini, is “most pronounced” during the summer. Polling error peaks at a nearly 6-point inflated edge for Democrats in late July and doesn’t begin coming down until late August.

The investigation says polls settle into an average error of an added 2.6 edge for Democrats on Election Day.

Three races: each in 2020, 2022, and 2024, illustrate the pattern.

The first is South Carolina’s 2020 Senate race between the late Republican Sen. Lindsey Graham and Democrat Jaime Harrison. Ruffini cites a Quinnipiac poll showing a tied race between the two and buoyed by a $57 million fundraising quarter, it seemed as if the race was neck-and-neck up until Election Day.

“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 kicked off a trend of neck-and-neck polls that continued almost until the very end. Graham would eventually win by 10,” Ruffini said.

Indeed, all three of Quinnipiac’s polls of the race showed statistical ties and a CBS/YouGov poll gave Graham just a 1-point edge in September.

Ruffini uses the 2022 Ohio Senate race in which Democrats pinned their hopes of picking up the seat of retiring Sen. Rob Portman on Tim Ryan, a long-time Congressman from eastern Ohio against now-Vice President JD Vance.

Vance overcame a crowded field in the primary to face off with Ryan following two previous wins by President Trump in the state’s presidential elections of eight points.

Ryan would lead polls as late as mid-September before Vance pulled away and kept the seat in the hands of Republicans.

“Ryan led in the vast majority of polls conducted in Ohio over the summer until being bested by JD Vance by 6 points,” wrote Ruffini. “The same pattern repeated itself in Ohio in 2024 when (Sherrod) Brown led in all but two pre-October polls in his race against Bernie Moreno.”

Ohio’s 2024 race featuring Sherrod Brown, who had been in the Senate since 2007, and Moreno followed a similar trajectory. Moreno led in just five polls of more than twenty in that contest.

Over the summer months of June, July, August, and September, polls gave Brown an edge of +2 to +6. Moreno would be elected with 50.1 percent of the vote to Brown’s 46.5 percent.

Ruffini’s research also offers a warning of midterm and presidential year polls.

He writes that in the Trump era, the Senate polls in presidential years have been biased towards Democrats and they remained that way. Yet in the midterms, the polls became more accurate but showed large biases towards Democrats in the summer months.