The former head of the Better Together campaign has called for Boris Johnson to stay out of the debate on Scottish independence.
In an interview with the Daily Record, Blair McDougall, who led the No campaign to a 55% victory in the 2014 referendum, agreed with Labour MP Ian Murray who said the Prime Minister poses a "greater threat to the UK than any nationalist does".
The strategist's comments come as Scottish independence looks to be one of the key areas heading into May's Holyrood elections, with the SNP holding a strong lead in the polls.
The party has outlined its road map to independence if a majority of supportive MSPs are elected, which includes plans to hold another referendum even if Westminster refuse permission, effectively daring them to take legal action to stop it.
The Prime Minister visited Scotland on Thursday, where he talked up the positives of co-operation within the UK in tackling coronavirus, while claiming the SNP uses independence as a diversionary tactic from domestic problems.
With Mr Johnson's personal ratings in Scotland consistently skewing negative, Mr McDougall told the Prime Minister not to give the SNP a foil.
"I'd say to him 'stop being the villain that the SNP want you to be. Step into the background and box clever'," he said.
"You should recognise that this is a battle that will be won or lost in Scotland."
Comparing former prime minister David Cameron's role to the current No 10 incumbent, Mr McDougall said: "(Mr Cameron) boxed clever and kind of stepped back from it.
"There is a distinct lack of that artistry from Boris Johnson where every intervention is briefed as being the intervention that will save the Union.
"If David Cameron understood that he was not the man who was going to save the Union, and that it was going to be saved in Scotland, Boris Johnson certainly isn't."
In the last 20 opinion polls on the subject, Scottish independence has been the favoured view when undecided voters are removed.