Gordon Brown is braced for a Labour rebellion today against moves to scrap the first-past-the-post voting system.
But the Prime Minister looks likely to win a comfortable Commons majority in favour of holding a referendum on electoral reform after securing the support of the Liberal Democrats.
But there is little parliamentary time to push the move on to the statute book. Mr Brown is preparing to challenge the Conservative leadership to support the measure – or risk losing other widely supported pieces of legislation.
Labour whips are desperately trying to win as large a majority as possible for the measure despite widespread hostility among the party's MPs.
As many as 40 Labour MPs are understood to oppose strongly Mr Brown's belief that first-past-the-post should be replaced by the alternative vote (AV), under which candidates are ranked in order of preference.
They believe the proposed new system would produce a result where the winner was the least unpopular, rather than the most popular, candidate. However, party sources believe that the rebellion can be limited to a hard core of about 20 Labour MPs.
Opponents of electoral reform are being told that they should vote against it during a referendum campaign, rather than in today's vote, which is only on the principle of holding a referendum and not on the merits of changing the voting system. Others are being given permission to absent themselves from tonight's vote.
The Conservatives are vehemently against ditching first-past-the-post. David Cameron yesterday repeated his charge that the Prime Minister was acting out of self-interest.
Mr Cameron said: "Look at what his idea of reform is – trying to fiddle the electoral system and introduce the alternative vote in a cynical attempt to save his own skin."
The Liberal Democrats will today table a proposal for the introduction of the single transferable vote, which is far more proportional than AV. But they will vote in favour of AV as a stepping-stone to proportional representation.
The Government is also likely to be supported by a handful of nationalist MPs, paving the way for a healthy government majority on the issue.
Mr Brown is keen for the margin of victory to be convincing to send a message to the House of Lords not to block a constitutional reform that affects only the Commons. If peers refuse, the scene would be set for a protracted period of parliamentary wrangling as the parties negotiate over which pieces of legislation become law before the election, expected on 6 May. Labour strategists believe they will be able to portray the Tories as the supporters of the status quo if the opposition thwarts moves to get the referendum promise on to the statute book.
They think the move will also be appealing to liberal-minded voters and could prove an olive branch to the Liberal Democrats in the event of a hung parliament.
The Conservatives retort that an electorate more interested in the health of the economy and the state of public services will be able to see through the Labour tactics.
Willie Sullivan, spokesman for the Vote for a Change campaign, which has called for a referendum, said that under the current system, the election result depended on 250,000 voters in marginal seats.
He said: "Polls have shown that people are prepared to break with the past."