New Democrats say they're now willing to support a national referendum on electoral reform.
The move appears aimed at forging a united front among opposition parties in favour of a proportional voting system, putting pressure on the governing Liberals to go along.
The opposition parties hold the majority on an all-party committee that is supposed to recommend an alternative to Canada's first-past-the-post voting system by Dec. 1.
Committee members have been holed up behind closed doors this week, attempting to reach a consensus which has so far proved elusive.
The NDP and Greens support a proportional voting system but Conservatives have said they will not support any change unless it is approved by Canadians in a referendum.
Liberal members have not tipped their hand on what voting system they prefer, but Prime Minister Justin Trudeau has in the past expressed a preference for a ranked ballot system.