Some polls showing a Clinton decline, including one by The Upshot, were conducted before the letter by James Comey was released. Most important, the polls taken before the letter were as bad for Mrs. Clinton as those conducted after it. Again, there aren’t many of these polls, but taken at face value there’s a case that Mrs. Clinton had nearly or even completely bottomed out by the time the Comey letter was released. Even if she had not, the trend line heading into the Comey letter was bad enough that there’s no need to assume that the Comey letter was necessary for any additional erosion in her lead.