A single Idaho poll narrows our margin, not our odds

A Bullfinch Group survey of 608 likely voters in Idaho, fielded August 16-19, puts independent Todd Achilles at 38 percent and Republican Sen. Jim Risch at 33 percent. Neither candidate is close to a majority in that poll: about 29 percent of respondents are recorded for neither man. No margin of error is attached to the survey in our records, and a five-point gap in a sample that size is a close result rather than a settled one.

Our model registered the poll and moved, but not much. The mean margin in Idaho is now R+18.7, narrowed from R+23.7 in our previous run earlier the same evening. The 5th-to-95th percentile range runs from R+29.8 to R+7.7, compared with R+34.7 to R+12.6 before. Risch's chance of winning is unchanged at effectively 100 percent, and the Democratic chance stays at 0 in 100.

That is the substance of the update: a five-point shift in the expected margin, with the probability untouched. We wrote on August 17 that Idaho polling had been scattered and that our model had moved toward Risch. The direction of the latest movement is the other way, and the scatter is part of why one survey showing Achilles ahead does not overturn a projected double-digit Republican margin.

The race matters to the national picture only at the margin. Our model gives Democrats a 48 percent chance of Senate control against 52 percent for Republicans, with a mean of 50.2 Democratic seats; Democrats need 51 seats for control and Republicans 50, because the vice president is a Republican and a 50-50 tie goes to them. Our generic-ballot average stands at D+6.7 across 25 polls in a 45-day window.