Deputy Prime Minister Nick Clegg will today launch a strident attack on his Conservative Cabinet colleagues, saying the party "cannot be trusted".
In a speech to the Scottish LibDem conference in Dundee, Mr Clegg will also suggest the Tories have changed little since the days of Margaret Thatcher.
And he will attack the Home Secretary Theresa May, following suggestions she is positioning herself as a future Tory leader.
Mr Clegg will also use his speech to accuse the SNP of failing to tell Scots the truth about the realities of independence. But it is his full-scale attack on the Tories that will raise eyebrows.
The LibDems have struggled in the polls since the party entered Coalition with the Conservatives in 2010.
In those early days, Mr Clegg even faced accusations that he was too friendly with Tory leader David Cameron.
Today the LibDem leader will say: "People in Scotland know that the Conservatives cannot be trusted to deliver a fairer society. Not on their own anyway."
And he will touch on the party's continuing reputation in Scotland.
"I know that the fact of going into Coalition with the Conservatives has been particularly controversial in Scotland," he will say. "The legacy of Margaret Thatcher's government lingers long in the memory."
His decision to attack Mrs May, rather than Mr Cameron, will also be seen as significant.
He will round on suggestions that the Home Secretary is keen to abandon commitments to the European Convention on Human Rights.
Such a move would "abandon the rights of British citizens just to satisfy the right of the Conservative Party", he will say. "She wants this option to remain 'on the table'.
"No chance. As I said last weekend, it won't be on the Cabinet table as long as I'm sitting round it."
Mr Clegg will also accuse the SNP of failing to be "forthcoming in telling the public the true consequences of independence".
"In public they say the economy will be strong and society will be fairer. In private, they know the oil won't last forever and they don't know how they'll pay for public services, defence and your pension."
In a move that will be seen as firing the starting gun on the 2015 General Election, he will also tell delegates to spread the word about the positives of their party. "I need you to explain it to people each and every day, from now, for the next two years and beyond. Tell them that only the Liberal Democrats have the values and ideas to build a better future."
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