A one-point margin in VA-1 leaves our forecast where it was

The only new number in Virginia's 1st District comes from the DCCC Targeting Team, which surveyed 519 likely voters September 15-17 and found Republican incumbent Rob Wittman at 47 percent and Democrat Shannon Taylor at 46. No margin of error is attached to the survey in what we have, and a one-point gap on that sample size is not a lead. The finding is that the race is close.

Our forecast did not move. The model gives Taylor a 38 in 100 chance of winning the seat and Wittman 62 percent, the same split as our previous run earlier Monday afternoon. The mean margin shifted from R+2.8 to R+2.7 — inside the noise of a single update. Our 5th-to-95th percentile range now runs from R+17.5 to D+12.1, against R+17.7 to D+12.0 before. That is a wide band, and it is wide because a district polled once this close can plausibly land several points either way.

The gap between a close poll and a 62 percent Republican chance is the gap between the survey and everything else the model uses. One survey of 519 voters, from a Democratic committee's targeting operation, does not on its own overturn a district-level estimate that starts Republican-leaning.

Nationally, our average of 29 generic-ballot polls over the past 45 days stands at D+7.5. The model puts Democrats' chance of winning House control at 97 percent, with a mean of 243.2 seats against the 218 needed. One caveat on that figure is ours: our simulation draws correlated error at the national, census-division, state and district levels, but we have no data to add a term linking demographically similar districts wherever they sit. Without it, the House control probability is marginally firmer than it should be. We cannot say by how much on this run.