Radical pan-democrats have failed in an attempt to recruit more members to the Alliance for True Democracy, the group campaigning for genuine universal suffrage in the 2017 chief executive election.
The Post understands that The Frontier, a group member of People Power, had applied to join but failed to get enough support in a vote by the alliance's members yesterday.
Mandy Tam Heung-man, secretary general of The Frontier, said the result was expected but regrettable.
"We just wanted to give a helping hand to the alliance in fighting for public nomination as the Democratic Party was leaving. People who oppose us should not put party interests over the fight for democracy," Tam said.
Two weeks ago the Democratic Party decided to leave the alliance in protest after radicals reneged on promises to back the alliance's reform proposal in Occupy Central's deliberation day last month.
Ivan Choy Chi-keung, a political scientist at Chinese University, said the pan-democrats feared the radicals were trying to increase their influence.
"The mutual trust within the alliance is already very fragile after last month's deliberation day, as some members are unhappy that the radicals hijacked it," he said.
"The trust would be further [compromised] if The Frontier joined in."
Lawmaker Cyd Ho Sau-lan, of the Labour Party, said it was hard to understand The Frontier's motives, given that most of the alliance's work - such as formulating its reform proposal - was over.
The Civic Party's Ronny Tong Ka-wah feared moderate pan-democratic voices would be drowned out if more radicals joined.
The Frontier was formed in 1996 and merged with the Democratic Party in 2008. Some members who refused to join the Democratic Party restarted the group in 2010 and it became a member of People Power in 2011.