Peter Mandelson has joined the fray as the struggle for Labour voters' support on the alternative vote intensifies.
With just ten days to go of an increasingly bitter campaign the former business secretary has called on Labour supporters to "use their noddle" by hurting the Conservatives with a 'yes' vote.
The Tories are united against the proposed alternative vote system. Labour has split in two on the issue, with many - including former New Labour ministers John Reid and David Blunkett - preferring to keep the first-past-the-post system currently in use.
Lord Mandelson has now advanced a partisan argument for coming up with a 'yes' vote. "Labour shouldn't ignore this chance to defeat the Tories on May 5th," he told the Independent newspaper.
"Labour supporters need to use their noddle and ask themselves why Cameron is fighting so hard for a No vote.
"He's fighting for his party's interests but also to protect his own leadership. Labour has a chance to inflict damage on both."
Former foreign secretary Margaret Beckett is leading the 'no' campaign for Labour today, as she joins forces with Tory culture secretary Jeremy Hunt to campaign in central London.
"With just days to go, it is more important than ever that we put aside party differences for the good of the country and that everyone comes out to vote against the unfair and expensive alternative vote system on May 5th," she said.
Pollsters say Labour supporters need to back AV by more than two to one if the Yes to Fairer Votes campaign is to emerge on top.
That seems unlikely - reflected in 'yes' campaign emails over the weekend calling on supporters to "fight back" against the 'no' campaign's negative tactics.
Complaints from Liberal Democrats about the tone of the campaign have dominated the Easter weekend and are now threatening the coalition government.
Ms Beckett added: "The increasingly hysterical statements of the Liberal Democrats running the 'yes' campaign have no place in this debate.
"I strongly urge people to look at the two voting systems in this contest and they'll see that our current system is simple, fair and decisive; whereas AV is an expensive and complicated political fudge."