Tony Blair has urged voters to resist the temptation to give him one final "kicking" in May's Scottish elections.
Mr Blair is on his third visit to Scotland in a week
The prime minister said he would be gone soon but Scotland would have to live with the "consequences" of an SNP government for many years.
But Nationalist leader Alex Salmond hit back saying on what would be his last visit to Scotland as PM, Mr Blair should apologise for the war in Iraq.
He added that two thirds of Scots did not trust the Labour leader.
In a speech during a visit to Glasgow, Mr Blair argued that times had changed since 1997 and the onset of devolution, and the case for the Union, had to be made in a different way.
He accused the SNP of copying Labour, which held a referendum before going ahead with devolution for Scotland.
"They mean to get independence the same way," said Mr Blair.
He said the SNP "know that if they can hold it three or four years down the line" it would give them a chance of "stoking up enough conflict with Westminster" to create a sense of grievance and make Scotland feel it had "no option" but to go with independence in a referendum.
Mr Blair warned: "From day one, from May 4, an SNP government would start the instability.
"Scotland would be on a track to separation. The SNP would be in the driver's cab with every intention of getting there.
"That is their reason for being. People might believe you can stop it but the SNP will be doing their best to keep it on track.
"Put it like this - if you really don't believe in independence, then voting SNP is at the very least a crazy risk to take."
However, Mr Salmond was keen to remind Mr Blair of his role in starting the war in Iraq.
The SNP leader said: "Four years on from this illegal and unnecessary war, it is clear that the quagmire in Iraq will be Tony Blair's legacy.
"The death toll continues to mount - both our own troops and Iraqi civilians.
"A Red Cross report this week concluded that the situation for ordinary Iraqis is getting steadily worse."
He added: "This is one of Tony Blair's last visits to Scotland while still in office, and he should take the opportunity to apologise for the war in Iraq.
"After all of the carnage and chaos, the least he can do is say sorry, which would help to draw a line under this bleak chapter."