In recent history no 3rd party candidates have come anywhere close to getting a substantial number of votes in the presidential elections. The last time a minor party was able to win a presidential election, was with Abraham Lincoln, in 1860. He was not truly campaigning under a minor party either; the Republican Party which he ran for had recently gained control of the congress and was becoming well established in the federal government. This is a key issue that 3rd parties must understand before they consider putting up a candidate for president.
The reason why 3rd party candidates don't win in presidential elections is not because they are unqualified; it's because the electoral system that we have in place creates enormous obstacles for them. One obstacle is that 3rd party candidates get unfair representation in political debates on television and another is that 3rd party candidates have trouble mobilizing their voters to get to the polls.
Despite what's against 3rd parties they still do manage to make some important contributions in our political system. One contribution is there ability to shift other party's views along the political spectrum slightly. Take the case of the 2000 and 2004 elections where Ralph Nader was running as the extreme liberal 3rd party candidate in the election. Green party supporters would argue that by Nader running