A close South Dakota poll leaves our SD-1 forecast where it was

A Lake Research Partners survey of 500 likely voters in South Dakota's 1st District, fielded August 26-30, found Republican Marty Jackley at 47% and Democrat Nikki Gronli at 43%. That is a close race in this one poll.

Our forecast did not move. Our model still gives Gronli a 0 in 100 chance of winning the seat, rounded, and Jackley 100%. Our expected margin is R+33.3, the same as in the run at 4:32 PM ET today. The edge of our range shifted slightly. Our model now puts the 5th-to-95th percentile outcome at R+47.4 to R+19.1, compared with R+47.5 to R+19.1 before.

The gap between this poll and our estimate is wide. Even the most Democratic end of our range, R+19.1, is far more Republican than the margin in the Lake Research survey. One poll of 500 voters, fielded more than a month before today, carries limited weight against the other inputs in our model. It nudged the range by a tenth of a point and left the central estimate unchanged.

The district has little bearing on the national picture in our forecast. Our model gives Democrats a 98% chance of winning a House majority, with a mean of 245.1 Democratic seats against the 218 needed. That figure is somewhat firmer than it should be. Our simulation correlates error nationally, by census division, by state and by district, but it does not link demographically similar districts in different places, because we lack the data to do so.