Ed Miliband has accused the Tories of using the SNP to distract voters from their record as he tries to kill off talk of post-election deals.
The Labour leader said the election was "not a clash of two nations but a clash of two visions".
His party has been attacked by the Tories over potential alliances with the SNP in a hung parliament.
The Conservatives said it was a legitimate argument because Labour "can't get a majority without the SNP".
And Labour former Scottish first minister Henry McLeish said Mr Miliband may have little choice but to negotiate with the Scottish Nationalists.
"At the end of the day, Ed is not going to exclude himself from being prime minister by not talking to anyone," he told BBC Radio 4's Today programme
'No child benefit cuts'
In an appeal on the last weekend before the 7 May poll, David Cameron urged people not to vote tactically, but to "vote for your preferred prime minister".
"Don't take a risk thinking 'I'll vote Liberal Democrat and hope I get the prime minister I want' or vote UKIP and hope somehow it emerges," he told BBC Breakfast.
"If you want me to carry on leading the country, making sure we have that stability and security in our economy, vote accordingly because we are only 23 seats short from that overall majority."
He also insisted there would be no cuts to child benefits as part of Conservative plans to shave £12bn off the welfare budget.
The prime minister spoke out as Mr Miliband continued to insist he would not lead a government if it involved any deal with Nicola Sturgeon's party.
In an interview with the Guardian, Mr Miliband accused the prime minister of having "entirely withdrawn from the central issues facing the country... in a bid to distract voters from the big choices".
The Labour leader also told the Guardian that 10 of his manifesto proposals would be introduced as bills by the end of May if he formed a government.
These include a "strong economic foundation bill" aimed at reducing the deficit, introducing a mansion tax and tobacco levy to fund the NHS; freezing energy prices until 2017; banning recruitment agencies from hiring only from overseas, and cutting university tuition fees in England to £6,000.
In other election news:
Mr Cameron is due to outline the Conservatives' manifesto for pensioners
Liberal Democrat leader Nick Clegg will set out plans for a new taskforce aimed at tackling youth unemployment aimed at helping 100,000 young adults
Labour and Plaid Cymru have clashed on whether the two parties could work together in a hung parliament in a BBC Wales' televised leaders debate
Mr Miliband has urged voters in Scotland not to "gamble" with the SNP, but the Conservatives insisted the SNP would be "the monkey on Labour's back".
However, Mr Miliband insisted: "What this election really comes down to is not a clash of two nations but a clash of two visions.
"Two different plans ideas about how our country succeeds."
He said the Conservatives would offer "huge tax cuts" for the "rich and powerful", whereas Labour thinks "Britain succeeds when working people succeed".
Mr Cameron is due to highlight his party's commitment to the "triple lock" on state pensions, which means they rise by whichever is higher out of inflation, average wages, or 2.5%.
He will say the triple lock - which Labour says it is also committed to - would take the annual pension to £7,000 a year by 2020.
The prime minister will also outline other previously announced proposals including the protection of benefits such as free bus passes and TV licences, and giving people more freedom to invest and spend their pensions.
He will suggest a Labour government "would mean a return to higher taxes, spending and borrowing and pensioners would be particularly vulnerable because many of them do not have the option of increasing their incomes".
Child benefit 'stays'
Labour has claimed the Tories plan to cut child benefit as part of their planned £12bn welfare savings.
The Tories say they will freeze working-age benefits, including child benefit, for two years. There is some speculation that freeze could last the duration of the next Parliament.
On BBC Breakfast, Mr Cameron was asked if there would be no increases to child benefit cuts if he resumed office.
The PM said: "That's right, I've said it stays as it is. I've been very clear on child benefit. I could not have been clearer."
He said: "It's very important for family's budgets to be able to plan, and they can know with me that it stays as it is - and we've been very clear about this."
He said "billions of pounds" would be raised by freezing in-work benefits, like unemployment benefit.
"I think that's right because we should be making sure you're better off in work," he said.