Officials will reveal results of a branch consultation of its 64,000 members in Wales on whether it should back the Welsh Secretary, Alun Michael, or his backbench rival, Rhodri Morgan.
The GMB has 6.2 per cent of the overall vote and its support could be crucial if constituency parties, Welsh MPs and assembly candidates are as evenly divided as predicted.
Mr Michael, the preferred choice of Tony Blair, is assured of the block votes of the transport and engineering unions, although Mr Morgan has the support of Unison, the only big union to conduct an individual ballot.
After a three-month campaign, union votes are being seen as crucial before the victor is declared in Cardiff tomorrow. Unions have a third of the overall vote, with MPs, assembly candidates and the party's 25,000 members in Wales also balloting as part of an electoral college. Opinion polls and ballots among smaller unions have been in favour of Mr Morgan, the Cardiff West MP, by about 3:1.
His team was buoyant yesterday after the Manufacturing, Science and Finance union backed him, the ninth to do so.