"Super seat" contender Frederick Fung Kin-kee said he was worried that People Power's call to cast blank votes could hinder pan-democrats' goal of winning three of the five seats.
Fung, of the Association for Democracy and People's Livelihood, expects a 16 per cent share of the vote will be sufficient to win one of the seats - in the functional constituency for district councils. That means the camp would 48 per cent of the votes to win three seats.
"If there are many blank votes, the pan-democrats will have a slim chance of winning the votes [to secure three seats]," he said.
Fung said it was possible to compensate for the loss of ballots from blank votes if there was a turnout of more than 50 per cent of the 3.2 million voters eligible.
Electors without a vote in another functional constituency can vote for the "super seats", so called because the winners will have a bigger mandate than fellow lawmakers.
People Power appealed to supporters not to vote in the "super seat" ballots or to cast blank votes in the September 9 election. The seats were created by 2010 political reforms, which it opposed.
Ivan Choy Chi-keung, a political observer at Chinese University, said the so-called 6:4 golden ratio - for every 10 ballots cast, six go to pan-democrats and four to Beijing-loyalist candidates - was unlikely to be maintained in the "super seat" race if the People Power supporters heeded its call to cast blank ballots.
Opinion polls show slates led by the Federation of Trade Unions' Chan Yuen-han and Democratic Party chairman Albert Ho Chun-yan enjoy a clear advantage, with independent Pamela Peck Wan-kam far behind.
Lau Kong-wah and slates led by Fung, Democrat James To Kun-sun and Starry Lee Wai-king of the Democratic Alliance for the Betterment and Progress of Hong Kong are in a tight race, with the margins on their share of the vote at around four percentage points - or 60,000 votes.
Meanwhile, a rolling poll by the University of Hong Kong's public opinion programme commissioned by Now TV estimates People Power's candidates will win 170,000 votes across the five geographical constituencies.