Gordon Brown is trying to intimidate Scotland into remaining part of the UK, Conservative leader David Cameron says.
Mr Cameron was responding to criticism from the chancellor
He wrote in the Scottish Mail on Sunday that the case for preserving the union of England and Scotland must be made "with the heart as well as the head".
He insisted there was "no question" the union had made both countries stronger.
Mr Cameron was responding to criticism from the chancellor, who has accused the Tories of risking the future of the UK by siding with nationalists.
The Tory leader said the chancellor was using rhetoric about the "fear of the economic consequences of going it alone" to intimidate Scotland.
And he denied Mr Brown's claims that the Tories had taken "anti-unionist positions".
This year marks 300 years since the Act of Union united the two countries' parliaments, and Mr Cameron said he would be celebrating the anniversary.
Gordon Brown is widely expected to be the next Labour leader
"But we need to be honest. For others, this anniversary will not be a cause for celebration," he wrote.
"With the majority of Scottish voters now favouring independence, polls show that the union is under serious threat."
Mr Cameron argued that Scotland needed a resurgence from the Conservative Party - which has only one MP north of the border.
He said people were disenchanted because of the "nature of the political forces" there.
He accused Labour of offering "big-state solutions and endless interference into people's lives".
And he said the Scottish National Party (SNP) offered "rhetoric on the dismemberment of the union" but lacked "intellectual coherence".
The SNP is tipped to do well in the Scottish Parliament elections in May - with the independence debate likely to crop up often in the campaign.
Liberal Democrat leader Menzies Campbell said: "Mr Cameron seems determined to put short term electoral advantage against the long term interest of the people of Great Britain.
"This is a reflection of the fact that his own party is dying on its feet in Scotland.
"You can be certain that if the Tories were making any headway North of the border Mr Cameron would be extolling the virtues of the Union. This is convenience politics."
In a speech to the Fabian Society on Saturday, Mr Brown accused the Tories and nationalists of "playing fast and loose" with the union.
He said there was now a dividing line in Britain between "those of us who are prepared to support the shared values of the union" and those who are prepared to "put the whole future of the union at risk".