538: Toomey's 95% chance of win
Wednesday seemed like such a good day for Joe Sestak. One poll had him down by 3 points in the Senate race to Pat Toomey, so did another, and a third had him within 5. But the same day Rasmussen's phones were in the field doing more polling, which placed Toomey up by 9.
What gives? Lefty poll expert Nate Silver at Five Thirty Eight/NYT has been using Toomey-Sestak as a test case to explain his election forecasts, where in Pennsylvania's case he says Toomey has a 95 percent chance of winning the race. How can the race be that much of a perceived lock on the first day of October?
But in Part 1, I explained why our intuitions can mislead us when it comes to something like evaluating Mr. Sestak’s chances. The data from recent elections, by contrast, suggest that even relatively small leads in the polls can be very difficult to overcome.
I have a database containing almost all polls conducted in all U.S. Senate and gubernatorial races since 1998. I say almost all because it excludes internal polls released by campaigns and other explicitly partisan groups, and it excludes Internet polls conducted by Zogby Interactive, which in my view are not scientific. My database also does not include polls for irregularly-scheduled special elections, like the one in Massachusetts earlier this year — only contests in November.
Mr. Toomey’s lead is around 7 points in the polls. How often have Senate and gubernatorial candidates with a 7-point lead in the polling average — with about a month to go in the campaign — fared in the past? Let’s construct about the simplest possible study around this:
. . . Senate candidates who have a lead of between 6 and 9 points in the simple polling average, with 30 days to go until the election — about where Mr. Toomey’s lead stands now — are undefeated since 1998. This isn’t quite as impressive as it sounds, since there are only seven such candidates in the database. But if we expand the scope of our study just a bit, it proves to be the norm rather than the exception. Senate candidates with a slightly larger lead in the polling average — one between 9 and 12 points — are also undefeated. Candidates with a slightly smaller lead in the polling average — one between 3 and 6 points — have a pretty good track record, with nine wins against three defeats.
Indeed, no Senate candidate with a lead of more than 5.5 points in the polling average, with 30 days to go in the race, has lost his race since 1998: these candidates are 68-0. (Martha Coakley in Massachusetts would have been an exception, but special elections, where the polling can be much more erratic because of lower turnout, are outside the scope of our study.)
NYT graphic