Markey leads by 21 in first new Massachusetts poll; our margin narrows slightly

A UMass Amherst/YouGov survey of 800 registered voters in Massachusetts, fielded August 5-12, puts Democratic incumbent Ed Markey at 51 percent and Republican John Deaton at 30 percent. That is a 21-point margin, with roughly a fifth of respondents not accounted for by either candidate.

The result did not change the shape of the race in our forecast. Our model still gives Markey a 100 in 100 chance of winning the seat, unchanged from our previous run earlier on August 18. What moved was the expected size of the win: our mean margin is now D+31.2, down from D+35.5, with a 5th-to-95th percentile range of D+20.2 to D+42.3. The new poll sits at the low end of that range, which is why the average came down.

In other words, this is confirmation rather than news. A single survey of registered voters showing a 21-point lead is consistent with a race our model already treated as settled, and the adjustment it produced is to the margin, not to the probability.

The national picture is where the uncertainty is. Our average of 26 generic-ballot polls over the last 45 days stands at D+6.7. Our Senate simulation gives Democrats a 47 percent chance of winning control and Republicans 53 percent, with a mean of 50.1 Democratic seats. Democrats need 51 seats for control and Republicans need 50, because the vice president is a Republican and a 50-50 tie breaks their way. Massachusetts, on these numbers, is one of the seats doing none of the work in that calculation: it is priced as certain, and the chamber turns on races that are not.