The Prime Minister said he backed Ms Davidson “every step of the way” as he attempted to boost the embattled leader.
He said Ms Davidson was the “ideal leader” for the Scottish Tories when he addressed their conference in Stirling.
With Ms Davidson facing internal criticism for her about-turn on devolving more powers to Holyrood, Mr Cameron also signalled his support for her new position on the constitution.
Ms Davidson won the Scottish Tory leadership contest on a ticket advocating that a “line in the sand” be drawn under the current constitutional settlement. Since then, however, she has asked a commission led by Lord Strathclyde to examine devolving more powers to the Scottish Parliament.
In a speech which included a staunch defence of the UK government’s welfare cuts, Mr Cameron said: “We will only succeed if we are in touch and in tune with modern Scotland and in Ruth you have the ideal leader.
“Ruth wasn’t born into the Conservative Party, she chose it. She understands that to win we’ve got to be a party for all of Scotland.”
As he endorsed Miss Davidson’s leadership, Mr Cameron strongly hinted his party should support more powers for the Scottish Parliament.
Mr Cameron said that the Tories should be a party focused on “securing Scotland’s place within a strong UK, but not afraid to look at how devolution can be improved”.
Mr Cameron’s keynote speech at the Albert Halls in Stirling will be seen as a boost for the head of the Scottish Tories, whose leadership has recently been the subject of much conjecture.
Her failure to land meaningful blows on Alex Salmond at Holyrood’s First Minister’s Questions has been a source of frustration for a Scottish Tory Party which has make no progress at the ballot box in recent years.
The establishment of Lord Strathclyde’s Commission has irritated some of her supporters who believe that the genie of more powers for Holyrood should be kept in the bottle.
The fact that the issue of constitutional change was not debated on the floor of the main conference irritated some activists. That irritation spilled over to the fringe events, where the idea of more devolution was discussed. Some malcontents among activists expressed their dissatisfaction that the issue had been relegated to venues away from the main stage.
At a meeting organised by the Law Society of Scotland, Alistair Orr of the Conservatives’ Stirling Association said: “I have been very exorcised by the fact that we have not been allowed a proper floor debate in conference about this issue. It is something to be deplored.”
Having arrived in Stirling to make his speech at about 10:15am, Mr Cameron was met by audience that struggled to reach the 200 mark. In a sign of the difficulties facing the party, which is struggling to find support, there were still some empty seats in the hall when Mr Cameron stood up to speak.
The referendum on Scottish independence was a major theme of Mr Cameron’s speech.
The Prime Minister pledged to fight for the Union against the independence movement, setting out a vision for a UK that was “pulling together, not pulling apart”.
“Together we’re unbeatable. United we’re unstoppable. The case is unquestionable … head, heart, body and soul. We will fight for the United Kingdom every step of the way.”
Mr Cameron’s devotion to the Union even extended to singing the praises of the former Labour chancellor Alistair Darling, who is leading the Better Together Campaign, and will appear at a No campaign event at the conference today.
The SNP pounced on an extract in which Mr Cameron remarked that when “one of your conference darlings is a Labour MP, you know you’re on to something.”
Angus Robertson, the SNP’s Westminster leader, said: “It says it all that the darling of the Tory conference is a Labour MP – and underlines that Labour have followed the Tory agenda on welfare cuts, and are joined at the hip with the Tories in the No campaign.
“Labour voters in Scotland will be appalled that Alistair Darling is being praised to the skies by a Tory Prime Minister – nothing better illustrates how Labour have got their priorities upside down on both the welfare state and the constitution.”
Mr Cameron was unrepentant on the UK government’s controversial welfare reforms and its drive to bring down the UK’s deficit. Defending the decision to bring in a benefits cap, Mr Cameron said the current system was “unfair” whereby a family on benefits could earn more than a family in work.
“Compassion isn’t measured by the size of the benefits you pay out or how angrily how defend the status quo,” Mr Cameron said. “No, it’s about helping people get back on to their feet if they possibly can.”
Cameron trails party in popularity for first time
David Cameron is less popular than his party for the first time, according to a poll.
The survey, conducted for former Conservative treasurer Lord Ashcroft, showed a “noticeable shift” towards dissatisfaction with the Prime Minister’s performance, while the Tories lost ground on key issues like crime, immigration, welfare reform and the economy over the early months of this year.
Lord Ashcroft blamed the Tory setbacks on the party putting people off by “talking amongst ourselves” about peripheral issues like Europe and gay marriage rather than focusing on voters’ core priorities.
He issued a warning to the party’s leadership that there was “no time to waste”, and said it was vital for Chancellor George Osborne’s spending review later this month to show that Tories are doing what the public want from them.
Overall, 37 per cent of those questioned said they would vote Labour, 27 per cent Tory, 15 per cent the Ukip and 9 per cent Liberal Democrat, giving Ed Miliband’s party a comfortable ten-point advantage, but a majority still said they would prefer to have Mr Cameron than Ed Miliband in Downing Street.
‘Greater tax powers a knee jerk reaction’
The split in the Scottish Tory party over whether to embrace more powers for Holyrood broke in to the open at the party’s Scottish conference yesterday.
Despite David Cameron’s apparent endorsement of Ruth Davidson’s bid to clear the way for greater devolution, a former party leadership candidate criticised moves to give the Scottish Parliament more teeth.
At a fringe meeting run by the Tory Reform Group, Margaret Mitchell said the Scotland Act, which will
transfer greater autonomy over income tax, was a “knee jerk reaction” to the rising fortunes of the SNP.
She argued Tory support for the Scotland Act has earned the party no extra votes in Scotland, so it should return to its stance of resistance to further devolution.
She said that leadership “naval gazing” was unproductive, although she remains sceptical about the direction in which Ms Davidson is taking the party.
Ms Mitchell said: “All the parties have signed up to devolution and there is a vacuum arguing for the status quo in terms of the powers that Scotland has now, so Scotland has no choice.
“It came about because of knee-jerk reaction to a minority SNP government.”