Following the death of party leader Jack Layton, the NDP began a leadership race.
Cullen announced his official candidacy on September 30, 2011.
In his opening speech, Cullen stated his vision for a new and progressive kind of politics, saying "now's the time for an honest discussion about the direction our politics is going in...and how we must change it for the better."
 Cullen also argued the choice between the economy and the environment is unnecessary, but that a green economy is a balance between creating jobs and protecting the environment.
He cast himself as a pro-business candidate
Cullen proposed that joint primary nominations should take place in Conservative-held ridings to determine the best possible local candidate to avoid vote splitting among 'progressives'.
Cullen is suggesting that each constituency association would have the choice of whether or not to run their respective campaign in this way, and that there would be no "top-down" directive to do so.
In his policy papers thus far, Cullen has also advanced a number of other ideas such as: creating a national public transit strategy, instituting carbon cap-and-trade pricing, putting a moratorium on new genetically modified organisms, redefining the Canadian Wheat Board, and holding a referendum on voting reform (Nathan supports Mixed-member proportional representation as currently utilized in New Zealand and Germany).

Cullen's candidacy was endorsed by three fellow NDP MPs, Alex Atamanenko, Fin Donnelly, and Brian Masse (NDP trade critic), former Toronto-Danforth MP Lynn McDonald, four BC MLAs, Robin Austin, Gary Coons, Doug Donaldson, and Norm Macdonald, former BC MLAs Corky Evans, Lois Boone, and Joan Sawicki, as well as Ontario MPP Taras Natyshak, and Manitoba cabinet minister Jim Rondeau.
Slam poet Shane Koyczan has also supported Cullen's candidacy, as well as author and former Ottawa city councilor Clive Doucet and author/explorer-in-residence/ethnobotanist Wade Davis.

Observers and commentators noted that Cullen's performances in the debates increased his profile.
Whereas at the beginning of the race Cullen was considered one of the "also-rans", he eventually was given considerably more attention, with the CBC, The Globe and Mail, Toronto Star, and National Post all running articles on his candidacy.

On March 24, the day of the convention, Cullen surprised many by placing third in the first round of voting with 16% of the vote ahead of Peggy Nash, Paul Dewar, Martin Singh and Niki Ashton, with Thomas Mulcair and Brian Topp in first and second respectively (30% for Mulcair, 21% for Topp).
He continued to hold third place after the second and third counts, eventually being eliminated at the third count with 24.6% of the votes (Mulcair had 43% of the vote, Topp had 31%).
He declined to endorse another candidate.
Following the death of party leader Jack Layton, the NDP began a leadership race. Cullen announced his official candidacy on September 30, 2011. In his opening speech, Cullen stated his vision for a new and progressive kind of politics, saying "now's the time for an honest discussion about the direction our politics is going in...and how we must change it for the better."  Cullen also argued the choice between the economy and the environment is unnecessary, but that a green economy is a balance between creating jobs and protecting the environment. He cast himself as a pro-business candidate
Cullen proposed that joint primary nominations should take place in Conservative-held ridings to determine the best possible local candidate to avoid vote splitting among 'progressives'. Cullen is suggesting that each constituency association would have the choice of whether or not to run their respective campaign in this way, and that there would be no "top-down" directive to do so. In his policy papers thus far, Cullen has also advanced a number of other ideas such as: creating a national public transit strategy, instituting carbon cap-and-trade pricing, putting a moratorium on new genetically modified organisms, redefining the Canadian Wheat Board, and holding a referendum on voting reform (Nathan supports Mixed-member proportional representation as currently utilized in New Zealand and Germany).
Cullen's candidacy was endorsed by three fellow NDP MPs, Alex Atamanenko, Fin Donnelly, and Brian Masse (NDP trade critic), former Toronto-Danforth MP Lynn McDonald, four BC MLAs, Robin Austin, Gary Coons, Doug Donaldson, and Norm Macdonald, former BC MLAs Corky Evans, Lois Boone, and Joan Sawicki, as well as Ontario MPP Taras Natyshak, and Manitoba cabinet minister Jim Rondeau. Slam poet Shane Koyczan has also supported Cullen's candidacy, as well as author and former Ottawa city councilor Clive Doucet and author/explorer-in-residence/ethnobotanist Wade Davis.
Observers and commentators noted that Cullen's performances in the debates increased his profile. Whereas at the beginning of the race Cullen was considered one of the "also-rans", he eventually was given considerably more attention, with the CBC, The Globe and Mail, Toronto Star, and National Post all running articles on his candidacy.
On March 24, the day of the convention, Cullen surprised many by placing third in the first round of voting with 16% of the vote ahead of Peggy Nash, Paul Dewar, Martin Singh and Niki Ashton, with Thomas Mulcair and Brian Topp in first and second respectively (30% for Mulcair, 21% for Topp). He continued to hold third place after the second and third counts, eventually being eliminated at the third count with 24.6% of the votes (Mulcair had 43% of the vote, Topp had 31%). He declined to endorse another candidate.
Nathan Cullen