David Cameron sought to occupy Labour's traditional territory of health as the parties kicked off a crucial week of campaigning.
The Tories indicated that they would move to end the "postcode lottery" for out-of-hours care by guaranteeing access to a local GP 12 hours a day, seven days a week if elected.
The promise emerged as the election battle got into full swing, with all three leaders due to unveil their manifestos over the next few days, and polls suggesting the result is still too close to call.
Labour will launch its platform first on Monday, with Gordon Brown promising "substance" rather than "pyrotechnics". Key policies such as guarantees on hospital treatment and extending paternity leave were already announced.
Asked in an interview with the Sunday Times whether the document would feature any surprises, he replied: "You will be surprised by the pro-business nature of the manifesto, the pro-enterprise nature of the manifesto, and pro-industry nature of the manifesto."
Lord Mandelson admitted that the Tories won the opening skirmishes in the media over tax and spending, after a slew of business leaders backed their plans for reversing the bulk of the Government's scheduled National Insurance hikes.
But the Cabinet minister accused his opponents of "confusing publicity with credibility", and insisted their flurry of policy announcements were starting to "unravel".
"Deficit reduction was apparently their number one priority before the election started," he wrote in a memo to party activists.
"People in the country know you don't get something for nothing. Common sense will tell them if you are being offered something that is too good to be true, it is, indeed, too good to be true."
Meanwhile, Mr Cameron sought to cement his centrist credentials by pledging to govern on behalf of "everyone in Britain". He told the Sunday Telegraph he would avoid the more "divisive" policies associated with the Thatcher governments of the 1980s, instead urging the country to "join together, act decisively and move forward with optimism".