More than anything else, the mood among Democratic Left members will influence the outcome of the talks between Labour and DL, due to intensify next month.
Some Labour figures now say a union of the two parties into a party to be called the Labour Party is inevitable. However, there are a number of different views within DL, and the outcome of the party's internal debate on its future is still not clear.
Last April it emerged - after two disappointing DL performances in by-elections won by Labour - that some in the party favoured a merger with the larger entity. However, a large section of DL continues to favour the party's continuance as a separate force.
There is strong resistance to the notion that the party's four deputies and its members would simply join Labour. The party's recent history, involving metamorphoses from the republican movement into an eastern European-style socialist party to a modern western European-style democratic socialist party, has been difficult. However, it has served to solidify the membership's attachment to the party and reluctance to see it subsumed into Labour.
Few policy differences now remain between the parties, although many Democratic Left members still see themselves as "more left-wing" than Labour, albeit in an ill-defined way. Labour also has a relatively conservative and predominantly rural wing, which is absent from DL. Some DL members still say they have a role in what one called at the weekend "keeping Labour honest" i.e. being around to shout "sell-out" - and reap the benefits - if Labour ever does an unfavourable coalition deal.
Within Labour it is personal considerations which are to the fore among those who are unhappy about the idea. Labour deputies such as Ms Roisin Shortall (Dublin North West) and Dr Pat Upton (Dublin South Central) believe they stand a high chance of losing their Dail seats should a merger or electoral alliance be agreed. Others who wish to regain seats lost last year, such as Ms Niamh Bhreathnach (Dun Laoghaire), Mr Liam Kavanagh (Wicklow), and Mr Eamonn Walsh (Dublin South West) also feel joining forces would hinder their efforts.
Enthusiasm among Labour deputies is also tempered by the knowledge that at least some of them would have to move from senior front bench positions to make way for DL deputies should the parties merge. Figures in both parties say that the only purpose of a "new formation" would be to provide a boost for both parties. "The whole must be greater than the sum of its parts," said deputies from both parties yesterday.
Otherwise, ambitious and capable individuals from both parties could find themselves losing out if the new entity were to go into a coalition government having failed to grow substantially. A new party or formation would have to make significant electoral gains to win sufficient Cabinet seats in a future coalition to keep everyone happy.
Many in the Labour Party see the present negotiations as an opportunity to eliminate what was for many years a troublesome rival, and absorb some new talent by getting Democratic Left simply to join the Labour Party. The problem for those in Democratic Left who are most keen on a merger is that internal Labour resistance may mean Labour may not be keen enough on joining forces to make serious concessions. Next month will see the most crucial phase of the talks. The parties have agreed to discuss nitty-gritty details such as how to resolve constituency squabbles between members of the two parties, and how to approach the issue of the allocation of portfolios and places on a party executive, should a union of the parties be the chosen option. By the end of September Mr Quinn and Mr De Rossa hope to announce agreement in principle to continue the talks.
Democratic Left's annual conference could then be asked to approve a deal in mid-November. And if this happens, Mr Quinn is understood to have told Mr De Rossa that he would hope to hold a special one-day conference to approve a deal before the end of the year.
Events have moved very quickly. It is less than a year since the idea of uniting the parties was first seriously floated. At a press conference to mark his resignation as party leader, Mr Dick Spring said he would like to see Labour and DL merge in the future. He favoured "closer co-operation with a view towards long-term unity".
Mr De Rossa responded by ruling out the idea of a merger. However, he and the newly-elected Labour leader, Mr Ruairi Quinn, agreed to set up a joint study group to seek to identify common ground between the parties on policy issues. That group began meeting in April.
At Mr De Rossa's insistence, the joint statement issued when this group was being established emphasised the "distinctive roles" the two parties had to play, lest this was seen as a step towards merger.
In April and May Democratic Left had a series of meetings of party members around the State to discuss their future. Labour had just won by-elections in Limerick East and Dublin North, while Democratic Left had polled well below expectations. It emerged that a number of those in the party favoured merging with Labour, although a very large section of the party was attached to the idea of continued existence as a separate party. Mr Pat Rabbitte TD, one of those seen as most open to contemplating a merger, was measured in his assessment: "The world is changing, a new millennium is around the corner, a lot of ice is breaking on the Irish political scene and any serious review of the future of the left in Ireland must take place in that context."
The parties are now on the same political ground and personal relations between them have improved greatly despite localised difficulties.
Mr De Rossa has spoken of "joining forces" with Labour through "the pooling of the physical and intellectual resources of both, which would acknowledge the broad similarity of our objectives and respect the diversity of our traditions". Whether and how this aspiration can be achieved will emerge within a few months.