"Unite the Left" talk always seems to revolve around the NDP and the Liberals, which routinely founders on the fact that the Liberals have always been a party of the Centre, not the Left. Anyone who wants to consider a partisan re-alignment on the Left should give serious thought to the prospects of uniting Canada's three broadly social democratic parties - the NDP, the Greens and the Bloc Quebecois.
All social democratic parties make compromises with capitalism (that's what makes them social democratic parties) and these three parties have made different compromises, with different capitalists. But, as extensive polling (not to mention personal experience) demonstrates quite clearly, the voting base of all three have very similar Left-Right views, clearly within the social democratic tradition.
Crucially, the three together have been polling above one-third of the electorate for years now. While partisan re-alignment would necessarily affect that, we are still talking about a plurality of all voters.
Obviously it would be hard to pull off. But it should not be unthinkable.