Mr Swinney dropped his guard at the end of a positive day at the party’s conference in Inverness, during which he laid the ground for a decisive victory today over the leadership challenger, Bill Wilson.
Buoyed by one of his best speeches as party leader and a successful hustings ahead of the crucial vote, Mr Swinney used language with echoes of some of the more extreme elements in Irish politics.
After the hustings, he said: "I want to make sure there are no roadblocks on the way to independence.
"If we have some uncertainty about why people have voted for the SNP, the unionist parties and the Brit establishment will put up every roadblock to stop us succeeding.
"We need to get into the driving seat in Scottish politics and the driving seat of Scottish governance and ask the big question - do you want independence, yes or no? And then tell the Brits to get off."
The SNP leader has always been careful to avoid such emotive language in the past, and he was accused of letting his mask slip to reveal "a little Scotlander".
Sources close to Mr Swinney insisted that his comments were merely shorthand for independence and had no deeper significance.
But David McLetchie, leader of the Scottish Conservatives, said: "This little Scotlander attitude has come out loud and clear. For him, nationalism is about being anti-everything - anti-English and anti-British - in his frustration he has let his mask slip."
A spokesman for Scottish Labour said: "Mr Swinney is prepared to say anything to placate the fundamentalists in his party. This time, he has gone too far. There is no place in Scotland for his brand of extreme nationalism. He has shamed himself, and he has shamed Scotland."
Mr Swinney will be re-elected as SNP leader today with a thumping majority over Mr Wilson, a little-known party activist. He is expected to take about 80 per cent of the vote from activists, giving him a clear mandate to continue his reforms - although he knows the bitter leadership battle has damaged the party and set back the SNP cause.
Mr Wilson came off worst in the hustings and was ridiculed by some delegates when he tried to argue that the SNP should not be "reasonable".
He told delegates: "I say to you, progress does not come from reasonable people. Progress comes from people who refuse to be reasonable, who stand up and say, this is our argument, this is our position."
Despite heckling from some activists, Mr Wilson went on to claim the SNP’s "nervousness" about its flagship independence policy had cost it lost votes and eight lost seats at the Holyrood elections on 1 May.
Earlier, in his speech, Mr Swinney rounded on his critics inside the party, warning of political oblivion for the nationalist movement if they were allowed to take over.
He was scathing about all those, like Mr Wilson, who believe in independence and nothing else, saying: "That strategy ends in only one place. It ends in the language of 90-minute patriots, of faint hearts and fearties, in blaming the voters for not being as principled and pure as we think we are.
"When you end up blaming the voters, you end up with a lifetime on the margins. We will never win independence from the margins of Scottish politics."
Mr Swinney lambasted party members and MSPs who have spoken out to condemn him for not believing strongly enough in independence.
He singled out one west of Scotland MSP, Campbell Martin, for a withering assault.
Mr Martin hit back at Mr Swinney. He said: "To be quite honest, quite frank, I don’t think John is the man who is going to lead us to independence. But that is for this party to decide."