Responsible politicians should be pragmatic about electoral reform instead of simply claiming the moral high ground, former Democratic Party chairman Albert Ho Chun-yan says.
His remarks came as the pan-democrat camp remains split over whether registered voters must be given the right to put forward candidates for the 2017 chief executive poll, so-called "public nomination".
"Some places allow public nomination, which is a good thing, and I hope Hong Kong will have it, too," Ho said.
But public nomination was not the only path to democratisation, he stressed.
Ho, who contested the 2012 chief executive election, said the party's bottom line was that the proposal must "in essence" fulfil international requirements for universal suffrage.
"This is the final battle: we would definitely not accept any proposal that involves a mechanism to screen out candidates," he said. "This is non-negotiable."
Accusations of being too moderate - stemming from the Democratic Party's support for the government's reform package for the elections in 2012 - have been revived by radical democrats.
Ho rejected such claims, saying a proposal on reform had to be judged by its outcome - whether it succeeded in enabling people with different political views to compete fairly in a meaningful race.
"Politicians should have a sense of responsibility but must not simply claim the moral high ground," he said.
"You might need to take the high ground sometimes, but in the long term … you have to take social responsibility into account and what it means to society to actually achieve reform.
"Sometimes if a chance is missed, it will be gone forever."
Ho also disagreed with criticism that the party's moderate stance was simply aimed at getting pan-democrats into the race and not representatives from a wider political spectrum.
"As long as we have an election without screening, it is never too late for people to establish new parties to run in the race," he said. "It is not only for the pan-democrats."
Ho said it was very unlikely that the party would again find itself the only faction of the pan-democrats that supported a government proposal on reform.
This time, it would be voting according to the results of the Occupy Central movement "referendum" in June, in which Hongkongers will choose their preferred proposal.
"If society as a whole endorses the [government's] proposal, then not only we but other pan-democrats would vote for it too," he said.
But if the proposal failed to secure Hongkongers' endorsement, the party would have no choice but to veto it - even if it did meet international standards of universal suffrage.
"It would be a pity [in that case] for the public to reject it, but we cannot violate the will of the people," Ho said.
He added that the party was determined to support Occupy Central's civil disobedience action if the government failed to offer a proposal that people were satisfied with.
"This is the deal-breaker," he said.