Mr Cameron said his party was being "honest and upfront" about their intentions and what the country could afford given the "vast" size of the UK's budget deficit and the need for spending cuts.
He said his commitment to protect NHS spending was "bolted down" and also reaffirmed the party's pledge to cut inheritance tax.
But there was confusion over policy on tax breaks for married couples, a long-standing priority for many in the party, after Mr Cameron appeared to say he could not give a guarantee to legislate in the next Parliament.
"It is something we want to do, it is something we believe we can do, it is something within a parliament I would definitely hope to do," he said in an interview with BBC political editor Nick Robinson.
"But I am not able today to make that promise because today we face this vast budget deficit."
But soon after the interview, the Tories released a statement saying they would "definitely" recognise marriage in the tax system over the course of the next Parliament and would give more details in "due course".
Labour said the Tories were in "disarray" over the issue, saying such a policy could cost £5bn to implement.
The Tories said there were a number of different options for taking the policy forward, costing different amounts, and they were studying the experiences of other countries.
The BBC's political editor Nick Robinson said the Tories had cleared up some of the confusion about their position but not said how they would proceed and were searching for a cost-effective way of doing it.
Earlier - unveiling the first part of a draft manifesto - Mr Cameron said the NHS would be his "number one priority", pledging a series of maternity reforms in England to "meet mothers' needs".
Mr Cameron promised to direct spending at more deprived areas and said he could improve the NHS while also tackling the deficit.
He said he would not promise anything he could not deliver, admitting "we are not able to give people absolute certainty on everything".
But he said the Tories would offer a "positive message" in contrast to Labour's "negative approach".
In what also looked much like the type of media conference seen during official election campaigns, Mr Darling launched a fierce attack on the Conservatives' spending plans.
"The Tories have made over £45bn of promises, but can barely explain how they can pay for a quarter of this," he said.
"This leaves them with a credibility gap of £34bn. These are not long forgotten promises from another time. All have been confirmed in the last two years. Most have been repeated in the last few months. You can't fight an election on a nod and a wink."
The Tories said Labour's figures were "flimsy", since they wrongly assumed they would reverse planned government tax rises such as the 50 pence tax rate on high earners.
The Lib Dems said neither party had a convincing plan to improve the state of the UK's finances.
"There is an unholy conspiracy," the party's leader Nick Clegg said.
"Both Labour and the Conservatives are not being straight with people about the difficult decisions required to sort out the public finances, get us out of the recession and rebuild a fairer Britain."
And the Institute for Fiscal Studies said the differences between the parties on tax and spending were "dwarfed" by the challenge facing all of them in getting to grips with the deficit.
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