Ed Miliband is a "very dangerous person" who is using a "con trick" to get into Downing Street, David Cameron has warned.
The Prime Minister delivered the stark message about the threat of Labour sneaking into power with SNP support as he made a frantic final push ahead of the election.
In an interview with the Press Association, the Tory leader played down his inability to pull ahead in the polls, insisting he always expected the contest to go down to the wire.
Rejecting criticism of the way he has conducted the campaign, Mr Cameron said he believed the public was engaged and predicted turnout on Thursday would be high.
"I always thought this would be a very tough campaign, a very close campaign - not least because of the 2008 crash and the recession, taken time to recover," he said.
"So we always thought it would be a late decision making campaign."
Mr Cameron said he "did not buy" the idea that he had failed to meet enough "real" people - arguing that Mr Miliband was the one scared of voters.
"I have travelled to every country in the United Kingdom, every corner of the UK, I have done social media things I have done big interviews. I think it has been quite a good mixture.
"I am glad we did do a leaders' debate right at the beginning. Last time there was not a campaign.
"This time at least we have been able to get round the country and talk to candidates in marginal seats in a much more meaningful way.
"Some people say, are people engaged? I think they are. I think people are very interested.
"I think the turn out is going to be quite high. People are thinking quite hard."
Asked if he had underestimated Mr Miliband - whom he has previously dismissed as a "joke" - the PM said: "I have never underestimated the threat he poses to Britain.
"He is very anti-business, very anti-enterprise, I think he is a very dangerous person to lead our country.
"If anyone has fought a safety first campaign, he is always behind a lectern, he is never out meeting the public, he doesn't seem to take questions from real people."
Mr Cameron refused to say whether he thought the incumbent prime minister was entitled to the "first shot" at forming a government if there was a hung parliament.
But he insisted he would do the "right thing" for Britain in that situation.
"I am not going to speculate about what happens after Thursday," he said.
"I would say last time you could see how I behaved, I put the country first, put a government together, made the right choice. People know with me I will put the country first - whatever gets Britain strong, safe secure government, I will do the right thing."
Mr Cameron said there would be a "real question of credibility" if Mr Miliband entered Number 10 without being leader of the largest party.
"This whole thing about a Labour government backed by the SNP in government people feel deeply uneasy about, because it would be a government held to ransom by a group of people that don't want the UK to succeed," he said.
"So there is a massive credibility problem, and that, I think, is the issue and people are expressing their concerns about that to me. I am saying if you want to stop that, here is the answer."
Mr Cameron added: "I think he has made a rather false promise because he keeps saying no deal with the SNP, but of course he does not rule out becoming PM on the back of SNP votes.
"I think people can see straight through that.
"I worry that the alternative to a Conservative led government is this recipe for instability uncertainty, insecurity and possibly chaos."
The Prime Minister also took a swipe at Nick Clegg, after a close ally of the Liberal Democrat leader claimed Mr Cameron had admitted to him privately that the Tories could not win an overall majority.
"I think they are doing it because they are being squeezed out, because fundamentally this is a choice now about who becomes prime minister of the country, me or Ed Miliband," he said.
"I think Nick Clegg is in danger of becoming a commentator on this election rather than a participant.
"He is now saying if you don't vote for me you are going to have a second election. Well I think a lot of people are going to say 'Nick, actually I won't vote for you, I will vote for a decisive Conservative government and then I definitely won't have a second election'."
Speaking to reporters on the Tory battlebus as it carried out a series of visits in the South West as part of a 36 hour campaigning blitz, Mr Cameron said of Mr Miliband: "Basically what he's doing is a con trick.
"You can see what he's doing: 'look at this strong language about no deals and no pacts, and ignore the fact that I can only become prime minister off the back of SNP votes'.
"The question he needs to be asked more directly is 'are you saying that if there was a hung Parliament, if Labour and SNP had a majority of votes, you wouldn't become prime minister?'. If asked that question, I suspect the answer is 'no, I'm not saying that'.
"Therefore he is prepared for that outcome. In that outcome you would be held to ransom vote by vote, deal by deal, budget by budget."
"At that moment he will in the eyes of the British people totally break with what he said about no deals and no pacts with the SNP. The last promise he made before the election will be the first promise he breaks after the election."
Mr Cameron denied suggestions that London Mayor Boris Johnson had been deployed too sparingly in the campaign.
The two men appeared together at a town hall style meeting in Hendon earlier. "Boris has been helping in the most positive way possible," Mr Cameron said.
"You can see today. People always long to write that there is some kind of dreadful competition ... there is not actually, we work very closely together and he gives me great support."
Asked whether he and Samantha had warned their children they may need to move out of Downing Street, Mr Cameron said: "They know that this is a close election, they know that Daddy is in a big fight for the Blue team, but we will deal with consequences when the consequences have to be dealt with."
And the premier insisted there was no part of him that would be relieved when he does leave Number 10 - which he has admitted will happen by 2020 in any case.
"Of course it is a very demanding job, it is a hugely satisfying and interesting job. I feel I have got plenty more to give," he said. "I feel totally up for it and very excited, but in the end the public will choose."