The main party leaders are criss-crossing the UK appealing to undecided voters in key seats as the election campaign enters its final two days.
David Cameron said an SNP-backed Labour government was a "chilling" and "calamitous" prospect.
Ed Miliband said Labour would not end up borrowing more than the Tories if he formed the next government.
Nick Clegg warned of a "second election before Christmas" if there was a minority Tory or Labour government.
With two days to go before polling day:
- David Cameron said he would "put the country first" if he failed to win an outright majority
- Ed Miliband said he did not accept an Institute for Fiscal Studies assessment that debt would be £90bn higher in 2019-20 under his plans than under the Conservatives' if he won the election.
- He also made a plea for people to vote Labour to "save the NHS"
- Nick Clegg said an EU referendum was not a coalition "red line" for the Lib Dems
- UKIP candidate Robert Blay was suspended after being filmed apparently threatening to shoot a Conservative rival while another of the party's candidates apologised for offensive remarks about a female journalist
- The SNP suspended two members of their party following scuffles at a rally held by the Scottish Labour leader in Glasgow
- The Green Party urged voters to "send a message" on climate change
- Lucy Powell, the vice-chair of Labour's general election campaign, denied suggesting Ed Miliband could break his election pledges.
- The Democratic Unionist Party said a commission on the union would be a "red line" in any post-election negotiations
Polls suggest the election is still too close to call, and in the final days the parties are focusing on their core messages amid speculation about post-election deals if there is a hung Parliament.
In an interview with the BBC, Mr Miliband was asked whether Labour would "borrow a lot more" than the Conservatives.
He replied: "No, I don't believe we would."
He said he did not agree with the IFS, which said last month the debt would be £90bn higher under Labour's plans than the Conservatives'.
"Well, I don't agree with that and I'll tell you why," he said. "The record of the Tories in this parliament is to say that they will deal with the deficit and then failed to do it...
"I'm saying that actually all of the evidence is the Conservative party, they can make the spending cuts but they can't cut borrowing as they promised."
And referring to his 8ft stone monument of pledges, he claimed his campaign vice chairman Lucy Powell had been wrong when she said no-one had suggested carving them into stone meant "he will absolutely not" break them.
The Labour leader also acknowledged for the first time that he might not win an outright victory on Thursday, by making clear his plan to abolish non-dom status for foreigners would be non-negotiable in any post-election deal.
He spoke as Labour was out campaigning on the NHS, publishing what it calls a leaked document suggesting 98 of England's 240 trusts are expected to have run up a combined deficit of £750m by next April.
Speaking in the target seat of Bedford, Mr Miliband said the NHS was facing a "financial bombshell", which would result in two-thirds of hospitals having to make substantial cuts this year.
Appealing to undecided voters as he seeks to improve on the 258 seats his party won in 2010, the Labour leader said the election would be "the closest we have ever seen in our history".
Mr Cameron told BBC Radio 5 live that the NHS had made "real progress" in the past five years.
"We have put the money in, we have got rid of bureaucracy, which has kept money on the front line," he said. "The key thing for the future is to make sure we have the strong economy that can support the strong NHS."
Analysis by political editor Nick Robinson
By 10 o'clock on Thursday evening the people will have spoken but the questions which will then follow look likely to be - "What on earth did they mean by that? Who actually won? Who has the right to govern?"
Unless the polls are wrong - which they very well might be - and unless there is a late switch in opinion - which there still could be - most players and pundits are now expecting an election that is too close to call and may produce a result which could allow for either David Cameron or Ed Miliband to become prime minister.
So, what is obsessing politicians of all parties behind-the-scenes is the debate about what a legitimate government would look like.
The Conservatives, which won 307 seats in 2010, are targeting seats held by Liberal Democrats, as well as appealing to UKIP supporters and Conservatives who might not bother to go to the polling station, in an attempt to win an overall majority.
While SNP MPs were perfectly entitled to make their voice heard in Westminster, Mr Cameron told the BBC, it would be "unhealthy" for a future government to be reliant on a party that "did not want the UK to be a success".
Mr Clegg said his party would do a "lot better" than commentators were suggesting as he launched a 1,000 mile "dash" from Land's End to John O'Groats, taking in key marginal seats in Cornwall, Somerset, South Wales and the Midlands.
Opinion polls suggest the party could lose up to half of the 57 seats it won five years ago.
Amid speculation about possible coalition deals in the event of another hung Parliament, Mr Clegg said the party with the "greatest mandate" in terms of seats and votes won should have the "space and time to try and assemble a government".
The Lib Dems, he told Radio 4's Today programme, would be prepared to talk to other parties - except UKIP and the SNP - in a "grown-up" way, saying they would be "guarantors of stability at a time of great uncertainty".
But he warned of a "shambles" if the Conservatives or Labour try to form a minority government and rely on the informal support of other parties to get their legislation through Parliament.
"The last thing Britain needs is a second election before Christmas," he said. "But that is exactly what will happen if Ed Miliband and David Cameron put their own political interest ahead of the national interest."
BBC News Timeliner: Meeting the voters
Meeting the voters on the campaign trail can be fraught with danger for any politician seeking election, as a quick delve into the BBC archives displays.
The best of BBC News' Election 2015 specials