David Cameron has said the Tories must stop making "knee-jerk" attacks on public service workers.
Mr Cameron has already highlighted his party's "green credentials"
As he celebrated six months as Tory leader, Mr Cameron said his party had to recognise that private firms could learn from the public sector.
He said: "Sometimes we have sounded a little hostile as if our approach is: there are too many of you and you are not working hard enough."
The GMB union's Brian Strutton said the Tory record made his words "laughable".
But another trade union leader welcomed Mr Cameron's message about the ethos of public services.
Mr Cameron's comments came as Tony Blair said reform was needed to maintain public support for public service investment.
In a speech to the National Consumer Council, the Tory leader said his party had too often given the impression it thought public sector workers were lazy and inefficient.
"Let's stop the knee-jerk attacks on public sector workers and focus on what really matters - improving the quality of service in our lives, whoever is providing it," he said.
Mr Cameron said banks, insurance companies and utilities firms were guilty of poor customer care and could learn from the public sector.
And he attacked politicians for trying to shift the blame on to civil servants, including Home Secretary John Reid, who recently said his own department was "dysfunctional".
Mr Cameron said: "When I hear MPs bashing bureaucrats - and I'm sure I've done it myself - I often think that what they're really complaining about is some idiotic bit of red tape that has landed on the desk of the blameless public servant.
"And when I hear ministers bashing bureaucrats - or declaring that their departments are `not fit for purpose' - I wish they'd have the decency to admit that very often it's their policies that are at fault, not the people who work for them.
"Instead of using public servants as scapegoats we should acknowledge their successes."
Mr Cameron said he wanted to trust people who worked on the frontline by giving them more power to get on with their job.
The speech follows a warning earlier this year from Tory former deputy prime minister Michael Heseltine about alienating Britain's six million public sector workers.
"If you want to win an election you might reflect that you have just dismissed academia, the teachers, the doctors, the police, the military, public servants and the voluntary sector from the Citizens Advice Bureaux, through to myriad organisations, all of whom, in one way or another, are supported by the public sector," Lord Heseltine told The Times.
But union leaders were divided in their reaction to Mr Cameron's speech.
The GMB's national secretary, Mr Strutton, said his words were just "political positioning".
"It is a bit laughable that David Cameron is trying to muscle in on this - we all know that it was the Tories who tried to destroy public services," he said.
But John Bangs, general secretary of the National Union of Teachers, said: "David Cameron has said he recognised the public sector ethos.
"There's a lesson there for the prime minister: if you blame your tools, as it were - the people who work in the public sector - instead of decisions that you have taken yourself, then you are going to leave a door open for the opposition," he said.