The FiveThirtyEight forecast had Mr. Obama gaining slightly on Wednesday, estimating that he had a 86.1 percent chance of winning the Electoral College on Nov. 6 — up from 84.7 percent in Tuesday’s forecast. This came despite the fact that it appeared there actually had been a modest shift back toward Mitt Romney in the polls even before the debate. In our “now-cast” — an estimate of what would happen if an election were held immediately — Mr. Obama’s projected margin of victory in the national popular vote had fallen by about one percentage point between Sunday and Wednesday. Our...