House at 97 percent for Democrats, Senate near even 46 days out

As of 6:32 AM ET on September 18, our model gives Democrats a 48 percent chance of winning Senate control and Republicans 51 percent. The mean Democratic seat total in our simulations is 50.2. Democrats need 51 seats for a majority and Republicans need 50, because the vice president is a Republican and a 50-50 chamber resolves their way. That asymmetry is why an average outcome slightly above 50 seats still leaves the two parties close to even.

In the House, our model gives Democrats a 97 in 100 chance of winning a majority, against 3 percent for Republicans, with a mean of 243.2 seats and 218 needed for control. Our simulation draws error at four correlated levels: one national, one for each census division, one for each state, and one for each district on its own. It does not include a term linking demographically similar districts in different places, because we have no cluster data for this run. The absence of that term makes our House control number marginally firmer than a fuller decomposition would leave it. We have not measured the size of that gap and are not adjusting for it.

Our generic ballot average is D+7.4, built from 28 polls over a 45-day window. Two new national surveys sit near or above that mark. RMG Research, for the Napolitan News Service, fielded September 8-15 and had Democrats ahead 51 to 43 among 2,000 registered voters, and 52 to 46 on its likely-voter measure. Big Data Poll, for the Public Polling Project, fielded September 13-15 and had Democrats ahead 47.8 to 38.1 among 2,947 registered voters, and 50.5 to 39.3 among 2,633 likely voters.

The largest movement in our Senate estimates was Minnesota, where the Democratic win probability fell 24.7 points, from 95 percent to 71 percent. Iowa moved the other way, rising 12.4 points to 43 percent from 30 percent. A co/efficient survey of 831 likely voters in Iowa, fielded September 14-16, had Republican Ashley Hinson at 46 percent, Democrat Josh Turek at 42 percent and other candidates at 2 percent. Smaller moves went against Democrats in Michigan, down five points to 66 percent; South Carolina, down 4.9 points to 13 percent; and Florida, down 4.5 points to 29 percent.

Elsewhere in Senate polling, co/efficient surveyed 799 likely voters in Alaska on September 14-17 and found a close race, with Republican Dan Sullivan at 49 percent and Democrat Mary Peltola at 46 percent. Quantus Insights surveyed 621 likely voters in Maine on September 14-15 and found Republican Susan Collins at 47.6 percent and Democrat Troy Jackson at 47.4 percent, effectively a tie. Wedgewood Polls surveyed 500 likely voters in Nebraska on September 14-16 and had Republican Pete Ricketts at 52 percent to 48 percent for the Democrat.