The votes that a third party candidate receives are often thought to come from the voters that would have voted for the party that is closest ideologically to the third party. Many believed that in 1992 and 1996 the votes that Ross Perot received came mainly from the Republican party, whereas the votes Ralph Nader received in 2000 came from Democratic voters. In all these instances the votes that the third party received, when combined with the votes from the party they are believed to have come from, would have changed the outcome of the popular vote.
Source
Nader for President http://www.4president.org/speeches/nader2000announcement.htm; Nader and La Duke http://cgi.ebay.ca/RALPH-NADER-2000-official-Green-Party-pin_W0QQitemZ7714346213QQcategoryZ33787QQcmdZViewItem; Perot bumper sticker http://www.4president.org/ocmi1996.htm; Perot in 92 http://www.geocities.com/dave_enrich/ctd/3p.major.candidates.html; Election results 1 and 2 http://www.trumanlibrary.org/museum/4years/election1948.htm. Created by Gary Keith and Mike Frost.