The former Chancellor and No campaign frontman spoke out during a visit to an Inverclyde engineering firm which is preparing to move to England if Scotland becomes independent.
As SNP Finance Secretary John Swinney again accused the No campaign of scaremongering, Mr Darling also used the visit to condemn those who have called for a boycott of one of Scotland's biggest travel agents, Barrhead Travel.
The company's founder Bill Munro warned that independence would be a disaster for the business.
Touring specialist engineering firm White House Products in Port Glasgow, Mr Darling rejected claims his campaign was in trouble after polls showed its lead was shrinking and an unnamed Coalition minister backed Alex Salmond's key proposal for an independent Scotland to share the pound in the formal currency union with the UK.
He said: "The arguments we are getting across are resonating with people.
"It is blindingly obvious the Nationalists are losing the economic argument.
"The critical reason for that is that people can see risk stamped all over what they are proposing."
Dismissing questions about his leadership, Mr Darling said the campaign was "the most important thing I've done in my political life."
He added: "I'm going to make sure we get the strongest possible result in September and I am devoting all my efforts to doing that.
The Better Together head also rejected comments by an unnamed UK minister who said "of course" an independent Scotland could negotiate a pound-sharing deal.
Describing the minister as "clearly someone who does not understand Scottish politics," he added: "There is no doubt that if we leave the UK we lose the pound."
He repeated his view that other parts of the UK would have a view on the policy but said it would not be put to the rest of the UK in referendum because the three main Westminster parties were all opposed.
He said: "There isn't going to be a currency union. The question does not arise."
Condemning the proposed boycott of Barrhead Travel, he said: "Frankly these attacks on business people shame Scotland.
"I really think it is important that whoever is doing this stops immediately.
"It doesn't matter what side of the argument you are on, you are entitled to have your say."
The firm Mr Darling visited makes hydraulic pumps. It was established 30 years ago and employs 22 people but would move to Northumberland if Scots vote Yes, managing director Alastair MacMillan warned, because 95% of its business is outside Scotland.
He said: "For us the prospect of the break up of the UK is dire, quite frankly."
He said he did not want to move the firm.
However, he added: "The trouble is that if you see your livelihood melting away you have to make difficult decisions.
"We have to win this battle. It is vital.
"This madness has to be stopped."
Mr Darling tried to draw a line under Better Together's most difficult spell during the campaign as the latest currency row reached Holyrood.
Meanwhile, answering a question from SNP backbencher Jamie Hepburn, Mr Swinney told Holyrood yesterday: "The opinion polls have clearly indicated that people in Scotland do not believe the bluff we've heard from the United Kingdom Government ministers and their allies in the Labour Party.
"What further demolishes the argument is the revelations at the weekend that the private chit-chat within the UK Government is that of course there would be a currency union.
"I think that helps people in Scotland to make it absolutely clear that the UK ministers that have tried to essentially scaremonger on this question have been found to have been seriously wanting in the way in which they have set out their arguments to the people of Scotland."