The leaders stuck to themes they have maintained for weeks in an effort to win over undecided voters, even though opinion polls have barely changed through the campaign. Cameron's Conservatives and Labor are neck-and-neck and neither is predicted to win enough seats for a majority in the House of Commons. Seven further polls are due to be published Wednesday.

Cameron warned of the dangers of a Labor government supported by the Scottish National Party; Miliband said a Tory government will only work for the "privileged few," and Clegg said only his Liberal Democrats can moderate the excesses of his two opponents.

"We have less than 48 hours to choose to build a country that works for all working people, not just a few at the top," Miliband said in a speech in Bedford, a district he hopes to win from the Tories after they won it by three percentage points in 2010. "The odds have been stacked against so many of the British people for far too many years now."

Polls show Labor and the Tories in a dead heat, with neither set to secure the 326 seats they need to govern alone. That leaves Cameron and Miliband dependent on other parties to govern, giving Clegg's Liberal Democrats, the current third party, and the Scottish National Party, which polls indicate will be third-biggest in the new Parliament, a say in who forms the next government.

SNP leader Nicola Sturgeon has said she won't prop up Cameron, but will support Miliband, leading Cameron to point to the dangers of allowing a party that wants Scotland to be independent into government.

"You don't have to imagine the chaos there'd be if Ed Miliband became Prime Minister," Cameron is due to say at a rally later Tuesday, according to pre-prepared remarks e-mailed by his party. "Nicola Sturgeon is on the television all day, everyday, telling us she plans to put Ed Miliband into Number 10, so that she could hold him to ransom every time there's a vote in the House of Commons."

Dangerous Alliances

Clegg has also warned of the dangers of allowing the SNP to support Miliband, or the U.K. Independence Party, which wants Britain to leave the European Union, exercising influence over Cameron. Only the Liberal Democrats can provide a "stable" coalition partner for Labor or the Tories, Clegg said.

"I know that no one is going to win outright," Clegg said Tuesday in an interview on BBC Radio 4's "Today" show. "The Liberal Democrats, much as we've provided a heart to a coalition with the Conservatives, would provide a brain to a coalition with the Labor party to make sure that they don't mess up the economy."

Clegg started a 1,000-mile bus tour on Tuesday, taking in Land's End in Cornwall, southwest England, and mainland Britain's northeasterly tip in John O'Groats, Scotland.

Cameron, speaking in northwest London on Tuesday, said "there is still time to determine the outcome of this election," but that in the event of an indecisive election result, "I will always put the country first."