October in an election year tends to be a bad month for incumbents seeking reelection. Going back fifty years, we have six decent comparisons to this cycle – 1956, 1972, 1980, 1984, 1996, and 2004. On average, the late September margin in the Gallup poll of registered voters closed by six to seven points in favor of the challenger. Only in the year 1956 did the incumbent expand his lead.

(I’ve excluded 1964 and 1976 because LBJ and Ford were not incumbents in the typical meaning of the word, and 1992 because Ross Perot’s jumping in and out of the race skewed the data.)