Prime Minister David Cameron will promise to keep Britain “on the road to a brighter future”, while Labour’s Ed Miliband will pledge “a government that will put working people first” reports BBC.
Lib Dem leader Nick Clegg will offer “stability and decency”.
Polls suggest no party will win enough seats for an outright majority.
BBC deputy political editor James Landale said politicians, pollsters and the media were struggling to read the election, leading many to focus on what might happen if there is an uncertain result.
“As such, Thursday might not be the end of the process,” he said. “It might just be the calling of the half-time whistle.”
The leaders have been criss-crossing the country in their battle buses as they attempt to drum up support ahead of Thursday’s poll.
Mr Cameron is heading to north west England, Scotland and the Midlands, while Mr Miliband is visiting Conservative-held marginal seats in the north of England.
Mr Clegg, who set off from Land’s End on Tuesday, is heading to John O’Groats through Scottish constituencies his party is hoping to retain.
The prime minister, whose Conservative Party won 307 seats in 2010, will renew his attack on the possibility of a minority Labour government propped up by the Scottish National Party (SNP) and try to dissuade voters tempted by the Liberal Democrats or UKIP.
The British people will make “their most important decision for a generation”, he will say.
“When you go into the polling booth tomorrow – know this: your vote really can make a difference.
“You can deliver a stable government and have a secure future, but only if you vote Conservative and for me as your prime minister.”