Labour and the Greens will consider doing deals in some electorates as part of a new agreement between the two parties to work more closely to unseat National at next year's election.
The left-of-centre parties yesterday unveiled a formal agreement described by Green co-leader Metiria Turei as "historic".
It includes clauses to work together in Parliament, as well as possibly on joint policies in some areas and a joint campaign.
The two parties would also not rule out a joint approach to campaigning in closely fought seats.
"The MoU [memorandum of understanding] allows us to have a conversation about whether we might do that kind of work," Mrs Turei said.
Such a move could be critical in marginal seats such as Auckland Central where Labour's candidate, Jacinda Ardern, came within 600 votes of National's Nikki Kaye in 2014, while the Green candidate took 2000 votes.
Withdrawing completely would be a step further than 2014 when the Green Party made it clear to supporters in some electorates they should vote for the Labour Party candidate.
However, it could cause friction between the two - the Green Party likes to stand candidates in areas where its party support is high to maximise that vote.
The agreement to work together effectively ends on election day and Mr Little would not rule out cutting the Greens out of any coalition post-2017 if that was required to form a Government.
"We both agree this is not a monogamous relationship," Mr Little said at a joint press conference in Parliament. "This is politics and this is MMP."
The Greens were left out by Labour in 2005 after New Zealand First leader Winston Peters made it clear he would prefer that they were not involved.
Mr Peters yesterday described the Labour-Greens deal as worthless.
"We don't like jack-ups or rigged arrangements behind the people's back," he told reporters.
"We will go into this election just ourselves, and our policies, seeking to change how this country is governed."
He would not say whether he would be willing to work with the Greens now.
"Why would we play third fiddle to a party we're past? When you come in where we come in, you don't play second fiddle to anybody. That's not a statement of arrogance, that's a
Mrs Turei said things had changed since 2005, including the leadership of both Labour and the Green Party.
"This is a new time, we have new leadership, we are beyond those times," she said.
The potential division of ministerial portfolios between Labour and Greens has been deferred.
Mr Little said Labour's finance spokesman, Grant Robertson, would become Finance Minister in a Labour-Greens Government, but no other positions had been settled.
Green Party co-leader James Shaw could fight for Economic Development, and Mrs Turei would seek a major social portfolio - it is understood she wants the education portfolio.
The Greens made a formal approach to campaign with Labour in 2014 but the two parties could not come to an agreement.
Green strategists were encouraged by polling which showed the parties' combined popularity was highest when they were working together, such as when they announced a
joint energy policy in 2013.
The next general election is scheduled to take place next year, no later than Saturday, November 18.
From the Left: Merger a powerful alternative
"The crisis consists precisely in the fact that the old is dying and the new cannot be born," wrote Antonio Gramsci, the famous Italian intellectual, "in this interregnum a great variety of morbid symptoms appear".
A little melodramatic, but it captures the thinking behind the memorandum of understanding between Labour and the Greens: both parties recognise the only future is a Labour-led government.
Of course, signing an agreement does not an election win make. But the signal is important: the two parties are moving past their old ambiguous relationship and preparing to form an alternative Government.
However, in John Key's New Zealand most Kiwis feel content.
The challenge for the new left is to offer people an alternative. Combining the "Parliamentary arm of the workers' movement" with the "Parliamentary arm of the environmental movement" is a powerful statement, but is it a reason for people to turn out and vote for it?
In a country where the Government is selling off state houses amid a homelessness crisis, where health and education are taking funding cuts in real terms, we desperately need an alternative.
For me, this is it.
- Morgan Godfrey is a Wellington-based political writer and First Union media officer.
From the Right: Time is finally right for clear-cut alliance of the left
Formalised ties between the Greens and Labour is an idea whose time has finally come.
Despite clamours from the left in the 2014 election run-up, a formal Green-Labour campaign then was a non-starter in a field cluttered by Kim Dotcom, Hone Harawira and Colin Craig. The kind of "open relationship" announced yesterday would have left too many questions about who else would sneak into government under the covers. However, the potential alternative government outlined yesterday instantly looks stronger.
John Key has often used the image of a many-headed left-wing monster in government to unnerve the electorate. The effectiveness of that scare tactic is diminished in 2016.
Forget "nice to haves" such as electability and charisma: right now, Labour lacks the talent in its caucus to assemble a credible Cabinet.
The injection of sensible heads like Shaw, Kevin Hague and the politically competent Metiria Turei as potential ministers will lessen that pressure.
Labour may rue giving up its wish to regain political superpower status in favour of a first-among-equals deal. But it needs both the Greens and New Zealand First to regain government. It's probably worth getting Winston Peters used to the idea.
- Ben Thomas is a public relations consultant, former National Business Review political editor, and former National Government press secretary.