Our South Dakota margin widens again while the odds stay put

The most recent survey of the South Dakota Senate race has it even. Impact Research polled 500 likely voters August 13-17 and found Republican incumbent Mike Rounds at 44 percent and independent Brian Bengs at 44 percent.

The other numbers in front of us are older and point the other way. Public Policy Polling, surveying 814 registered voters November 13-14, 2025, had Rounds at 44 percent and Bengs at 31 percent. The same survey tested Rounds against an unnamed Democrat and had him at 52 percent to 30 percent.

Our forecast has not moved in any way that matters to the outcome. It gives Rounds a 98 in 100 chance, with a 2 percent chance for Bengs and no measurable chance for a Democrat. The prior run, from earlier the same afternoon, put Rounds at 100 percent. What did change is the spread. Our mean margin is now R+24.2, out from R+20.2, and the interval around it is much wider: our model's 5th-to-95th percentile range runs from R+42.5 to R+5.4, against R+30.8 to R+9.6 before. In other words, the model is now less confident about the size of the result in either direction while remaining close to certain about who wins it.

Nationally, our model gives Democrats a 49 percent chance of winning Senate control and Republicans 50 percent, with a mean of 50.2 Democratic seats. Democrats need 51 seats for a majority; Republicans need 50, because the vice president is a Republican and a 50-50 tie goes to them. On the House side, our model gives Democrats a 93 percent chance of a majority, with a mean of 240.1 seats against the 218 needed. That House figure carries a caveat of our own making: our simulation correlates error nationally, by census division, by state and by district, but we have no cluster data to correlate demographically similar districts across the map, so the control probability is marginally firmer than a fuller treatment would leave it.