After nearly two weeks of battling for the support of MPs, the two began their six-week drive for the 250,000 grassroots party votes. Immediately, Tory chiefs were threatened with legal action over claims that tens of thousands of paid-up party members could be denied a vote.
Mr Davis travelled to his old university, Warwick, highlighting his non-Oxbridge background and stressing he could reach out to disillusioned former Tory supporters in the Midlands and the North. Mr Cameron visited an inner-city London radio station in an effort to highlight his appeal to young urban voters.
The battlelines started to emerge between the candidates on their first day of campaigning. Mr Davis told a packed meeting of students he was opposed to university top-up fees. "I don't think I would have gone to university if there had been top-up fees and tuition fees," he said.
Mr Cameron had told BBC Radio that, "regrettable though it is", he would retain the fees, which will be introduced next year.
The difference over a key issue, which could influence middle-class parents and Tory voters paying for their children's university, could harm Mr Cameron.
The rivals also struck contrasting notes on tax and investment in public services when they made a joint appearance on Radio 4's Today programme. Mr Cameron said he supported channelling any extra cash in the economy into schools and hospitals as well as into lower taxes.
He said: "I don't think anyone wakes up and thinks, 'Gosh, I wish the state was smaller today than it was yesterday'."
Mr Davis said tax cuts should be used as a tool for producing the economic growth that would in turn benefit services The candidates are also likely to clash over such issues as healthcare vouchers, immigration and the importance the party should give to Europe. But they have agreed that references to drugs use should be off-limits.
Mr Davis said: "For the next six weeks, I am not going to answer any questions on drugs, policy or otherwise, because I am simply not going to have this debate dominated by this issue."
His declaration pleased the Cameron team who had privately accused supporters of the shadow home secretary of stoking up the subject. Mr Cameron has refused to discuss any experience he may have had of drugs before he became an MP in June 2001.
The candidates yesterday met party chiefs to discuss the campaign timetable up to 5 December, the closing date for votes to be received. They are expected to hold between six and eight hustings meetings each in all regions of the country.