Welsh Labour will today vote on whether to form a coalition with Plaid Cymru.
First minister Rhodri Morgan has been accused of risking political "suicide" over the issue and admits he will have to quit if he loses.
However, he argues the power sharing deal, developed as One Wales, will allow Labour to carry out its manifesto pledges.
Labour emerged as the single largest party in the Welsh assembly after the May elections but, with 26 out of 60 AMs, were eager to form a coalition.
Initial offers were rebuffed by both Plaid Cymru and the Liberal Democrats - both of whom then considered forming a rainbow coalition with the Tories - and Mr Morgan resigned himself to running a minority government.
Now more than two months after the election, Labour and Plaid Cymru have agreed the terms of a coalition, fleshed out in the 43-page One World power sharing document.
Speaking to BBC Radio Wales this morning, Mr Morgan urged Labour members to "vote with their heads" and back the deal.
He said: "I understand the very strong emotions involved - I hope they will see this as a historic opportunity to deliver Labour's manifesto."
"In order to deliver 100 per cent of Labour's manifesto, what part of another party's manifesto do we have to swallow - that's really the proposition that we are putting to the party conference."
Mr Morgan continued: "I think most people on the Labour side realise that if Labour is the senior partner in a coalition and Plaid is the junior partner, that is very much better for Wales and for the delivery of Labour's manifesto than having Plaid as the head of the coalition with Tory ministers in the middle of it and Liberal Democrats supporting it as well, which is the alternative."
Labour will vote on the proposal at a special conference in Cardiff today, while Plaid debate the offer in Ceredigion tomorrow.
The majority of AMs have backed their leader, although Westminster Labour MPs are wary of Mr Morgan forming a coalition with a nationalist party.
AMs Lynne Neagle, Karen Sinclair, Ann Jones and Irene James have, however, broken ranks and accused Mr Morgan of risking political suicide.
The Labour vote takes place in two stages: The first half is split between the 40 constituency parties, 14 county parties and 17 women's forums and the second part is cast by 16 trade unions and other affiliated associations.