Nelson joined the Green Party of Manitoba in 2003, and was acclaimed as its leader at the party's annual meeting in November 2005. The position had been vacant since March, when Markus Buchart resigned as the result of ongoing divisions within the party. Nelson advocated reaching out to older and more conservative Manitoba residents, rather than relying on left-wing campus activists for the party's membership. She also advocated proportional representation, and said that the Greens would only run candidates in a small number of ridings in the next election.