Unite has not taken sides in the referendum campaign, as this would split the union - some 40% of whose Scottish members voted SNP in 2011 - "almost down the middle", said Mr McCluskey. And he said that Unite would continue to operate on both sides of the border even if Scotland votes to go it alone.
Despite polls consistently predicting a comfortable victory for the No campaign on September 18, he made clear that he is not ruling out a last-minute swing in favour of independence, similar to that seen in the Canadian province of Quebec in 1995.
With Labour's national poll lead over the Tories shrinking to as little as a single percentage point following last month's well-received Budget, Mr McCluskey suggested that the prospect of a second term as Prime Minister for Mr Cameron could be enough to propel Scotland's voters into the Yes camp.
Speaking at a Westminster lunch, the Unite boss said: "Remember we are talking about a nation that has kicked the Tories out of Scotland - there's only one (Conservative) Westminster MP up in Scotland.
"If, come September, the opinion polls down here are demonstrating that perhaps the Tories might get back into power, then that might have an influence on which way the Scottish people vote.
"They may take the view 'I'm not putting up with another five years of Tory policies, we may as well vote for independence and take our chance now'."
Mr McCluskey said that the rise of the Scottish National Party presented a challenge to Labour leader Ed Miliband, who needed to show that he was "on the side or ordinary Scottish workers" to counter the appeal of Mr Salmond's party.
"There's certainly a challenge for the Scottish Labour Party to demonstrate that they are on the side of ordinary Scottish workers, because Alex Salmond at the moment is charging up Labour's left wing and is seen by many people as more radical than the Labour Party," said Mr McCluskey.
"There's a lot that's going to happen between now and September, and that's a challenge for Ed Miliband to demonstrate that he is on the side of ordinary working people."