Scottish Conservative leader Ruth Davidson made her second keynote speech of the year yesterday, developing the idea of bringing more powers to Holyrood.

That will mean setting out details of any proposed changes in the party's General Election manifesto before the independence referendum in September 2014.

In a strategy clearly designed to head off independence, a group of experts will draw up plans for where devolution should move next. It is also a form of answer to the SNP, who are fond of recounting how the Conservatives promised "something better", if Scots rejected devolution in 1979, but which the Thatcher era significantly failed to deliver. (On the contrary, it heralded a decade of austerity and steep industrial decline.)

The problem with such rhetoric is that it struggles to sound either relevant or credible. Nobody should forget that the Scottish Conservatives attracted more than 50% of the popular vote in 1955. But in standing alone against popular support for devolution, the party succeeded only in appearing to be not merely unScottish but anti-Scottish. In 2010 the party failed to make a breakthrough in Scotland, despite big advances in middle England and the south. Recent opinion polls suggest the party now commands about 13% support from the Scottish electorate, so a Tory manifesto faces challenges to be relevant to the wider public.

Ms Davidson's claim that a mature Parliament should have significant tax-raising powers along with control of spending is neither radical nor new. Former Labour MSP Wendy Alexander first raised this issue after Scottish devolution became a reality in 1999. The Tory leader's new-found enthusiasm for US-style federalism has long been a policy associated with the Liberal Democrats.

There is a certain logic in Scottish Tories adopting such a standpoint because, regardless of which side wins the independence referendum, there are going to be arguments between those who see their main objective as tackling social injustice and those committed to cutting taxes and spending to kickstart growth. Initially, at least, as Nationalists have privately admitted, an independent Scotland would face difficult choices.

The problem for the Tory leader is that the position she now espouses so passionately is diametrically opposed to that on which she stood for the leadership. It is unlikely to please her party faithful, many of whom voted for her precisely because of what she then referred to as making "a line in the sand" over devolution. It will also infuriate those who supported her opponent, Murdo Fraser, whose political clothes she now appears to have filched and who has become something of a prophet in the desert.

Though Ms Davidson presents her ideas in the context of creating a distinctive identity for the Scottish Tories, the likelihood is that this departure suits David Cameron very nicely. Faced with pressure to tackle both the West Lothian Question (Scottish MPs voting on England-only laws) and the Barnett Formula (for working out Scotland's block grant), the idea of tearing up the latter and introducing a system of self-funding for Scotland looks most attractive. The problem is that someone who has moved through 180 degrees on such a major issue in such a short time appears to lack credibility.