Knowing that exit polling has historically overestimated the Democratic vote and knowing how much the final regular polling in the 1980 race understated Ronald Reagan’s support compared to Jimmy Carter, it is worth looking at what the final poll results said in other presidential election years.
The facts show a similar trend in a pro-Democratic direction almost uniformly. Historically speaking, pollsters have underestimated how many people would vote for the Republican presidential candidate:
Writing at National Review, reporter Jim Geraghty quotes an anonymous pollster who provides a helpful review of past polling data:
In 1992, Gallup’s final poll had Clinton winning by 12 percentage points, he won by 5.6 percentage points. In late October 1992, Pew had Clinton up 10.
In 1996, some reputable pollsters had Clinton winning by 18 percentage points late, and Pew had Clinton up by 19 in November; on Election Day, he won by 8.5 percentage points… In 2004, pollsters were spread out, but most underestimated Bush’s margin. (2000 may have been a unique set of circumstances with the last-minute DUI revelation dropping Bush’s performance lower than his standing in the final polls; alternatively, some may argue that the Osama bin Laden tape the Friday before the election in 2004 altered the dynamic in those final days.) In 2008, Marist had Obama up 9, as did CBS/New York Times and Washington Post/ABC News, while Reuters and Gallup both had Obama up 11.