Andrew Little will go up against David Cunliffe and Grant Robertson to contest Labour's leadership.
Mr Little confirmed this morning he would stand, after signalling over the weekend that he was thinking about it.
In a statement he said his background as head of the EPMU and former Labour Party president showed he had the required skills to drive through change and lift performance.
"There are three immediate issues to deal with: creating greater cohesion across the caucus, rebuilding the relationship between caucus and the Party and, most importantly getting the process under way to listen to the voters who have abandoned us."
Mr Little's union background could work in his favour with the affiliated unions getting 20 per cent of the vote.