MPs from Labour, the Scottish National Party and the Liberal Democrats will unite to launch the case for proportional representation (PR), as an alternative to first-past-the-post.
Charles Kennedy and Robert Maclennan of the Liberal Democrats will share a platform with the prominent SNP politician Margaret Ewing and the Labour "fair votes" campaigner Richard Burden at a meeting organised by Make Votes Count (MVC) the pro-PR campaign group.
"This shows that politicians from all parties are prepared to work for electoral reform," said a spokesman for MVC.
But officials from a trade union opposed to electoral reform will descend on the conference to try to tempt delegates away from PR.
The quasi-evangelical exercise by the Amalgamated Engineering and Electrical Union, Britain's third-biggest union, is likely to infuriate the Liberal Democrat faithful who have traditionally been in favour of voting reform.
The union officials want to expose splits in the party on the issue, and will try to persuade delegates to sign a petition opposed to Alternative Vote (AV) - a system which means people can mark a second and third choice on their ballot paper.
"It's part of our tactics to destroy this coalition that is pro-PR," said a senior AEEU official. "The first salvo is at the Liberal Democrat conference where campaigners are going down to try to recruit delegates for first-past-the-post."
Paddy Ashdown, the Liberal Democrat leader, is expected to make the case for a "fair voting system" which is "broadly proportional" in his conference speech.
A spokesman for the Liberal Democrats said: "[We] are united in favour of introducing a fair voting system. The AEEU are wasting their time if they think they can stop Liberal Democrats from backing reform."
But as reported last week in the Independent on Sunday, Mr Ashdown has indicated that he would accept a diluted proportional system known as the Alternative Vote Plus.
Some Liberal Democrat delegates, who do not regard AV Plus as broadly proportional, will voice their opposition to this move, which they regard as a compromise.
At Labour's conference later this month, Cabinet ministers Mo Mowlam and Robin Cook will publicly back proportional representation at a fringe meeting.
Next month Lord Jenkins, the former Labour chancellor, will present the findings of his commission on electoral reform.
The Cabinet is currently divided about whether to change the system used to elect MPs.Reuse content