But even if Alabama’s special election were just a normal Senate campaign with normal candidates, a lead in the low single digits would be far from secure. Simply put, Senate polling has not been especially predictive over the past 10 cycles. Among the 2,075 Senate polls in the FiveThirtyEight database that were taken within 21 days of an election, the average error has been 5.1 percentage points. And that has been fairly consistent across cycles. The 2016 Senate polls featured an average error of 5.2 percentage points.
The average error tends to understate the chance for a large miss, however. The true margin of error (i.e., the 95 percent confidence interval) is significantly wider. For the polling in Senate campaigns since 1998, it’s closer to +/- 13 percentage points for any individual poll. There hasn’t been a single public poll taken in Alabama since the Republican primary runoff that has given either Jones or Moore that large of a lead.