The leaders of five of opposition parties are due to appear in a live election debate later.
Labour leader Ed Miliband, UKIP’s Nigel Farage, SNP leader Nicola Sturgeon, Leanne Wood of Plaid Cymru and the Greens’ Natalie Bennett will take part reports BBC.
The BBC contest, hosted by David Dimbleby from central London, begins at 20:00 BST.
Looking ahead to the debate, Mr Farage said people had “underestimated” Mr Miliband.
He added: “What he has shown in this campaign is he is no fool and he is pretty tough, and if I am going to score points off him tomorrow night I will have to be good to do it.”
The BBC debate, which follows the launch of the parties’ election manifestos, is the latest in the schedule of election programmes agreed after months of wrangling between the parties and the broadcasters.
BBC assistant political editor Norman Smith said the stakes would be highest for Mr Miliband, who risks being “ganged up” on by the other leaders.
He predicted the dynamic between the Labour leader and Nicola Sturgeon will be the “most fascinating” element of the debate, given the “profound problem” Labour is facing in Scotland.