But leading Labour sources last night dismissed suggestions that the agreement gave any kind of commitment to back proportional representation (PR).
If Labour is elected, an agreement expected this week would provide for an electoral commission to be set up, with a one-year deadline in which to deliver its report on the detailed terms of a referendum.
The idea is that a Labour government would then have plenty of time to hold a national ballot and, if the voters called for the existing first- past-the-post electoral system to be dumped, enact legislation.
Theoretically, that could mean this year's election would be the last to be held under the current voting system. The Lib-Lab deal was spearheaded by Robert Maclennan for the Liberal Democrats, and Robin Cook, the shadow Foreign Secretary, who supports PR.
But, according to Labour leadership sources, Tony Blair's fundamental hostility remains as strong as ever, and there are real fears that premature moves could threaten party unity.
The delicacy of the Lib-Lab talks was also shown yesterday by Lord Holme, the Liberal Democrats' election campaign manager, who unceremoniously rejected a hint by Lord Jenkins, his party's leader in the House of Lords, that Labour and the Liberal Democrats might, at some point, merge.
The former Labour cabinet minister told a conference on Saturday that he would not rejoin his former party, before adding: "That, however, does not preclude a merger where it's a marriage of true and settled minds, as in the 1988 SDP/Liberal merger."
Lord Holme told BBC television's On the Record: "Nobody can exclude anything in politics, but it is certainly not on the agenda now ...
"Certainly, sitting here today, approaching this crucial election, when we're fighting on a very different platform from Labour, I absolutely exclude any possibility of a merger with the Labour Party as being on the agenda now or at any foreseeable future that I can see."
He said that one of the reasons the Liberal Democrats were so enthusiastic about fair voting was because they wanted to give people more choice, not less. "I'd much rather have a political system where you had people who are Liberals, people who are socialists, people who are Conservative, competing, and I think that would be far healthier for our democracy."
But Lord Holme - who is extremely close to Paddy Ashdown and his party leadership - said he was not completely confident that the Lib-Lab talks would deliver agreement.
"These talks are still going on," he said. "But let's agree for the sake of a useful discussion that it does seem very likely that there will be agreement sometime in the next few days, or weeks. I think, personally, that the idea of a commission to identify a fair voting system that could then be put to a referendum, I think it's a very good idea. I think it's a constructive proposal to come out with."
The difficulty faced by the Liberal Democrats and Labour is that they are currently vying for some of the same votes, and Mr Ashdown has been deliberately painting his party as the more radical alternative with a programme that includes redistributive tax hikes for the well-off, with the cash being redirected to about 500,000 low-income taxpayers.
They do not want to be seen as two parties on the verge of merger, or even two parties that are cooking up a post-election deal on electoral reform. Just as there are many Liberal Democrats opposed to pacts or deals with Labour, there are many Labour activists who are diehard opponents of electoral reform.
Even if he supported electoral reform, and there are the strongest doubts about that, Mr Blair would need to tread carefully; and he is.Reuse content