Nader ran as an independent.
Nader broke from the Green Party due to a philosophical difference with David Cobb on what strategy the Green's should pursue. Cobb was very concerned about Greens voting in swing states and urged people to vote their conscience.
http://en.wikipedia.org/wiki/David_Cobb Cobb stated his intention to run a campaign focused on building the Green Party and to pursue a "strategic states" or "smart states" strategy that would take into account the wishes of Greens in each state, and which otherwise would focus on states that traditionally are "safely" won by the Democratic candidate, or "safely" won by the Republican candidate, with a large margin of victory. Such so-called "safe states" are also referred to in campaign literature as "neglected states" because the Democratic and Republican candidates traditionally put most of their campaign energy into more competitive "swing states." Cobb's campaign said that, in each state, the campaign would aim to follow the wishes expressed by Greens in that state. While some of Cobb's erstwhile supporters urged swing state residents to vote for Democrat John Kerry in order to stop the re-election of President George W. Bush, other Cobb supporters encouraged votes for Cobb and LaMarche in all 50 states and the District of Columbia. The candidates themselves used the phrase "vote your conscience," campaigning both in swing states such as Wisconsin and safe states such as California.