Former Greens leader Bob Brown has endorsed Simon Sheikh to represent the party in its bid to unseat Liberal senator Gary Humphries in next year's federal election.
The ACT Greens membership will gather in Canberra on Wednesday night to hear from all those seeking the minor party's preselection for the Senate as well as for the lower house seats of Fraser and Canberra.
As the former director of lobby group GetUp!, Mr Sheikh has by the far the highest profile of all the candidates.
But he has received some criticism for seeking nomination after having only recently joined the party and moved to Canberra.
The three-week preselection ballot is already in process and will close on Monday, December 3.
Greens leader Christine Milne has already welcomed Mr Sheikh's willingness to join her team in federal Parliament, and now her predecessor has done likewise.
Dr Brown has given Mr Sheikh the nod on his candidate statement that will go to all party members.
''Simon's work for a more progressive and environmentally focused Australia has been outstanding,'' the former Greens leader said.
Neither Dr Brown nor Senator Milne will have a vote in the preselection ballot, but their opinions are highly regarded by the local Greens.
Only about 10 per cent of the 600 or so ACT Greens members are expected to turn up to Wednesday's meetings, but most of the membership will cast a vote before the ballot closes.
Long-time Greens member Kate Hamilton has described Mr Sheikh as a celebrity candidate and she wants the party to instead preselect her for the Senate campaign.
''I grew up in Canberra and feel a deep sense of connectedness to the country and the people,'' she said on her candidate statement.
''I have a compelling vision for Canberra as a dynamic, sustainable, growing economy.''
When contacted by The Canberra Times, Mr Sheikh welcomed Dr Brown's support. ''At the end of the day it is the members of the ACT Greens who will decide who is worthy of their trust and I look forward to continuing to engage with them,'' he said.