Labour's biggest affiliated union, the EPMU, has official thrown its weight behind former national secretary Andrew Little in the Labour leadership after failing to reach agreement in last year's contest.
The national executive unanimously backed Mr Little who was national secretary for 11 years and before that an employment lawyer.
But it is a recommendation only for a ballot which is secret and the rules ban bloc voting.
It will be a big boost to Mr Little's campaign. He is likely to get more union support, tacit if not explicit from some of the seven affiliated unions.
In the leadership contest last year between David Cunliffe, Grant Robertson and Shane Jones, the EPMU was divided with Auckland generally favouring Mr Cunliffe and Wellington generally favouring their home town candidate in Mr Robertson.
Overall last time, Mr Robertson won just 17.3 per cent of the union vote against Mr Cunliffe's 70.77 per cent and Mr Jones' 11.92 per cent., but Mr Robertson had more of the caucus.
Members of the Service and Food Workers Unions are able to vote individually; others are delegated. Mr Robertson won 35.61 per cent of the SFWU vote last time and he is hopeful of picking up more this time.
This time, Mr Robertson faces three opponents, Mr Little, David Parker and Nanaia Mahuta.
Endorsements will take on a new importance with voting starting today and the first of the candidates meetings happening in Wellington on Wednesday.
Mr Robertson has had the public endorsement of former Deputy Prime Minister Sir Michael Cullen, Jacinda Ardern, Kris Faafoi, Megan Woods, and Ruth Dyson.
David Parker has had a public declaration of support from former president Mike Williams, former general secretary Mike Smith, businessman Selwyn Pellet and Tahu Potiki.
Mr Little said he was not seeking endorsement but was pleased to get any.
Electing a leader
Affiliated Unions get a total of 20 per cent of the leadership vote divided as follows:
• Engineering Printing and Manufacturing Union 33.33%;
• Service and Food Workers' Union 21.92%
• Meat Workers' Union 19.55%;
• Dairy Workers' Union 13.71%;
• Rail and Maritime Transport Union 6.06%;
• Maritime Union of NZ 3.41%;
• Central Amalgamated Workers' Union 2.02%.