New Arkansas poll trims Cotton's projected margin, not his odds

A J.L. Partners survey of 803 likely voters in Arkansas, fielded August 14-18, put Republican Sen. Tom Cotton at 51% and Democrat Hallie Shoffner at 35%.

That 16-point margin is narrower than what our model had been carrying, and the model moved accordingly. Our current run has a mean margin of R+19.8, against R+23.4 in the previous run earlier on August 19. The 5th-to-95th percentile range also tightened, from R+34.5-to-R+12.4 to R+30.6-to-R+8.9. Every outcome inside that band is a Republican win.

The probability did not move, because there was nowhere for it to go. Our model gave Cotton a 100% chance of winning before the poll and gives him a 100% chance now; Shoffner's chance stands at 0 in 100. A single survey showing a double-digit Republican lead in a race the model already treated as settled changes the expected margin and nothing else.

The margin matters mainly as an input elsewhere. Our national average of the generic congressional ballot, built from 25 polls over a 45-day window, stands at D+6.7, and the Senate picture is close: our model gives Democrats a 47% chance of winning a Senate majority and Republicans 53%, 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 goes to the Republicans. Arkansas is not one of the seats that decides that; on our numbers it is one of the seats already accounted for.