Polling companies and media organizations sometimes argue that including third-party candidates in a poll produces an inaccurately high result for that candidate.
In the era of modern polling, there have been seven third-party presidential candidates included through the final polls in early November. Only one time did the difference between the final poll and the actual election result exceed 2%, and that was when Perot over-performed the final poll by five points in 1992.
Not once has a third-party candidate been included in polls, and then under-performed the final poll by more than the margin of error.
2000 Nader -Final Poll: 4% Result: 3%, Error +1%
1996 Perot -Final Poll: 7% Result: 8%, Error -1%
1992 Perot -Final Poll: 14% Result: 19%, Error -5%
1980 Anderson -Final Poll: 8% Result: 7%, Error +1%
1968 G. Wallace -Final Poll: 15% Result: 14%, Error -1%
1948 H. Wallace -Final Poll: 4% Result: 2%, Error +2%
1948 Thurmond -Final Poll: 2% Result: 2%, Error 0%