The Tories remain "the nasty party", Labour's deputy leader has said as she ended her party's annual conference.
Labour must "reach out and work with everyone", Mrs Harman says
"I think people want a prime minister who listens and gets on with the job, so they and their families can get on with their lives," Harriet Harman said.
She asked why would anyone want the "weak" David Cameron as prime minister.
And she promised a "clear choice" at the next general election - but did not comment on mounting speculation that a snap poll might be held this autumn.
Mrs Harman said her party wanted "every one" of its councillors and MPs "back after their next elections".
"We are counting on you to see that happens - and they must be joined by more," she told supporters.
Referring specifically to the Conservatives, Mrs Harman said: "The truth is the Tory party is still the nasty party, and I think David Cameron is too weak to change it."
She described Labour's need "to reach out and work with everyone who wants a fair, prosperous and peaceful Britain".
"We must change the way we do our politics and we are changing the way we govern. That means listening - not lecturing," she added.
"When people who have previously voted Tory instead vote Labour, and when people who have previously been members of the Tory party want to join us, that is a great thing - and we warmly welcome them."
She said that across Britain there were people "whose hopes for the future depend on a Labour government".
"We will not let them down," she promised.
Mrs Harman, the MP for the Camberwell and Peckham constituency in south London, has been Gordon Brown's deputy for three months, with both having taken up their roles in June.
She was taking over the end-of-conference speech which John Prescott has used for most of the past decade to rally the party faithful.
The Conservative Party said it would not be commenting on Mrs Harman's remarks.