Markey leads by 23 in new UNH poll; our Massachusetts forecast is unchanged

A University of New Hampshire survey of 556 likely voters in Massachusetts, fielded September 17-21, puts Democratic incumbent Ed Markey at 53 percent and Republican John Deaton at 30 percent, with 9 percent for other options. That is a 23-point margin, wider than the 19-point UNH lead this site wrote up on August 27.

The new number did not move our forecast. Our model gives Markey a 100 in 100 chance and a mean margin of D+37.6, identical to the mean in our previous run earlier on September 24. The 5th-to-95th percentile range shifted by a tenth of a point, from D+20.7 to D+54.4 before the update to D+20.8 to D+54.5 now. In practical terms, one 556-person poll showing a 23-point lead is less favorable to Markey than our average but nowhere near close enough to change the conclusion, and our range already spans outcomes on both sides of it.

That makes this a confirming poll rather than a new piece of information. Every Massachusetts survey our model has taken in has pointed the same direction, and the only question our numbers now address is the size of the margin, not the winner.

The contest matters for the Senate arithmetic only as a component of it. Our model puts the mean Democratic seat total at 50.6 and gives Democrats a 53 percent chance of winning control against 46 percent for Republicans, with 51 seats needed for a Democratic majority and 50 enough for Republicans because the vice president is a Republican and a 50-50 tie goes their way. A race our model treats as settled is one of the seats already inside that total; the uncertainty in the national picture sits elsewhere. Our national generic-ballot average stands at D+8.3 across 30 polls over the past 45 days.