Tory leader David Cameron has said he intends to present an "exciting and inspiring" alternative to Labour, amid speculation about a 2008 election.
Mr Cameron said he welcomed the challenge of a new Labour leader
Mr Cameron said, after ten years in government, he doubted Labour's new leader, Chancellor Gordon Brown, could provide "the change Britain needs".
He said the Tories had moved to the centre ground and he would now focus on a "relentlessly positive agenda".
Mr Brown takes over as prime minister from Tony Blair on Wednesday.
In his first speech as party leader on Sunday, Mr Brown appointed Douglas Alexander his election co-ordinator, prompting speculation there may be a general election as early as spring 2008.
Mr Cameron told BBC Radio 4's Today programme he was "looking forward to the challenge" of a new Labour leader.
"I've got a very important job to do, which is to give the British people an exciting and inspiring alternative to a government that I believe has failed," he said.
"And that's what I am going to focus on, a relentlessly-positive agenda."
Asked about a newspaper poll suggesting Labour was ahead of the Conservatives, for the first time since October, Mr Cameron said public opinion had been tested at the local elections, where his party won 40% of the vote.
He told the programme there had been three "very big changes" that had to be made to the Conservatives, when he took over as party leader - putting stability ahead of upfront tax cuts, improving the NHS for all rather than opt-outs for the few and expanding opportunity "for all" in education.
"The party has changed - it is in the middle ground, it is challenging for power," he said.
"It is much more relevant to people's concerns today in terms of really fighting the fight on health and education and crime."
He said, in the long term: "Fundamentally we will have a more socially mobile and fairer society if we have stronger families, we will have stronger families if we back marriage."
Mr Cameron said he would work alongside Mr Brown if he did "the right thing", saying the Tories had already supported the government's education bill.
But he said: "There are changes we need and my issue with Gordon Brown is I don't believe he can provide the change Britain needs, not least because he's had ten years in the job to do some of the things on school discipline and on basic standards of education which, just haven't been done, and they badly need doing."