Australian Senator Christine Milne unexpectedly resigned as leader of the small but influential Greens Party on Wednesday, raising questions over the future of an important political bloc outside the two mainstream parties.
Milne, who took over the party from founder Bob Brown in 2012, quickly built on its early successes by aggressively peeling voters from the main opposition Labor Party's left flank on issues such as climate change and asylum seekers.
The Greens chose the largely unknown Senator Richard Di Natale, a doctor from Victoria state who was elected to parliament in 2010, as her replacement.
"Richard Di Natale has been elected unopposed as the Australian Greens leader," Senator Rachel Siewart told reporters directly after the closed-door vote.
Milne's deputy Adam Bandt, a nationally known lawmaker from Melbourne, was surprisingly dropped from the leadership, while Senator Larissa Waters and Scott Ludlam were elected unopposed as co-deputy leaders, Siewart said.
Milne's resignation leaves the party without a proven leader less than two years from general elections. That vote will likely be seen at least partly as a referendum on the Greens' long-term viability as more than just a party of protest.
"It is with a mix of optimism, pride, excitement and sadness that I am resigning the leadership and leaving the Senate," Milne said in a statement.
"My decision to resign today is one I made with my family," said Milne, who is from the island state of Tasmania.
Milne has been a mainstay of left-wing politics since she was elected to the Tasmanian state parliament in 1989. She graduated from the federal Senate in 2004, where she served as Brown's deputy from 2008 until his resignation.
Despite early scepticism over her ability to replace the charismatic Brown, she led the party from strength to strength in national and state elections.
"Christine is the greatest champion of climate change action in Australian politics," Brown said in a statement.
Her resignation came just weeks after state elections in New South Wales in which the Greens scored their best-ever performance, building on anger among farmers over coal-seam gas mining to capture two traditionally right-wing seats.
Milne has been one of the nation's most vocal critics of conservative Prime Minister Tony Abbott. Di Natale will have a difficult first task of rebutting the government's annual budget next week.