By Carole Walker
Political correspondent, BBC News
David Cameron needs a big swing for his party to win a majority
David Cameron goes into this election ahead in the polls, but the bumps and dips of recent months have shown how fragile his lead could prove to be.
The Tory leader outscores Gordon Brown when it comes to being seen as nice and likeable. But the polls suggest he has not yet convinced the voters it is time for a Conservative government.
Even some in his own party believe he has to do more to explain what the Conservatives stand for.
After Mr Cameron became Tory leader in 2005 he said it was time to let sunshine win the day. Then the gloom of recession prompted a very different message - the age of austerity.
The Tory leadership's call for tougher, swifter action to tackle the government deficit may help reassure the markets but it worries some voters.
His opponents are warning that Mr Cameron is planning drastic cuts which will undermine the fragile economic recovery.
He is also under pressure to spell out more details of where he believes the axe should fall.
Mr Cameron has had to steady Tory nerves after a series of wobbles and confusion over some policies and the row over the power and tax status of the millionaire donor, and deputy party chairman, Lord Ashcroft.
His message for this election will be the choice facing voters - change with the Conservatives or five more years of Mr Brown.
But Mr Cameron knows that, in the uncertain economic climate, people may be nervous of opting for a new direction.
The Tory leader has set out to convince voters that he is a modern family man, in touch with ordinary voters despite his privileged background.
His wife Samantha will be on hand to support "Dave". But his relative inexperience and his judgement will be tested in the crucial televised prime ministers' debates.
While Mr Brown has played an important role in each of the last three elections, Mr Cameron - despite his backroom role in 1992 and 2005 - will be at the heart for the first time of the daily firestorm of a campaign fought out in a 24-hour media whirl.
And the Tory leader faces an electoral mountain. To win an overall majority, the Conservatives need a bigger swing than they have achieved in modern times.