As McCarten said: "My enemy's enemy is my friend."
The Alliance came within 1,000 votes of winning Tamaki. Act came even closer in the Taranaki-King Country. Both times Labour was beaten into third place.
And now Labour leader David Cunliffe has appointed McCarten his chief of staff. Both the left and the right are ecstatic.
The two sides of politics agree that McCarten's appointment signals a hard swing to the left.
The one thing McCarten doesn't do is compromise.
The left believe he will motivate hundreds of thousands of non-voters. The right believe that McCarten's strategy and style will drive centre voters to National.
He split the Alliance asunder and forced Helen Clark to an early election rather than play along with Clark's safe and centrist politics.
To keep McCarten on board Cunliffe must take Labour back to its socialist roots. There can be no more wish-wash from Cunliffe. Not if he's serious about keeping McCarten.
Both left and right agree Labour MPs are in for a fright. McCarten can't stand time-servers and fence-sitters. The Labour caucus is chock-a-block with both. Cunliffe, by appointing McCarten, has fired a rocket at his own MPs.
Where the left and right disagree is on McCarten's impact. The left believe he will motivate hundreds of thousands of non-voters. The right believe that McCarten's strategy and style will drive centre voters to National.
My pick is that it will all blow up with a bang. Cunliffe already lacks the support of his MPs. Plus the Labour MPs strategically want to hug the centre. McCarten, and now Cunliffe, want radical.
McCarten's response is always to bang heads. He will be reading the long-serving Labour MPs the political equivalent of the Riot Act: back the leader, get with the programme or get out.
But these very MPs have already seen off McCarten twice. They are nothing if not tough and cunning. McCarten's appointment makes left-wing politics way more interesting.
Where's the popcorn?