The former chancellor urged the "quiet but resolute majority" of Scots to play their part.
Mr Darling was speaking at a rally of hundreds of Better Together activists in Glasgow, where he called on people to say "no thanks" to independence.
With just over three months left until the vote, Mr Darling said: "Our opponents have spent months trying - and failing - to come up with evidence to support their goal of separation.
"And now with just 100 campaigning days left, the Nationalists are running out of arguments - and they are running out of time.
"So I suspect in the remaining weeks they will give up on the evidence and simply go for the emotion.
"That is where we, the quiet but resolute majority, each have a crucial role to play.
"Together in the days ahead we must, and we will, meet and match them in emotion as surely we have mastered them on the evidence."
The Labour MP also stressed that "with 100 days to go, the terms of trade have changed" as a result of the proposals from the pro-Union parties to hand more powers to Holyrood.
"It is now clear that a No vote will bring more powers to Scotland within the UK," he said. "Now all three of the Scottish parties backing a No vote have put forward broadly similar proposals for further powers."
While he said there is "much that divides us on other issues", he argued that when it came to the constitution, Labour, the Conservatives and the Liberal Democrats are "all now pretty much on the same page".
Mr Darling pledged: "I want to use these 100 days not to see Scotland divided further but to bring together most of us in this nation around a common vision of Scotland leading the United Kingdom after September 18, not Scotland leaving the United Kingdom.
"I want to use these 100 days to plan for Scotland's positive, possibility-rich future as part of the United Kingdom and with the substantially enhanced powers for the Scottish Parliament which all three parties have committed to.
"It is a vision that the overwhelming majority of us want.
"And so when voters go to the polls on September 18 I want every voter to understand that within the United Kingdom change and progress is coming to Scotland, underpinned by the commitments of all three parties."
Mr Darling added: "The Nationalists will be offering the negativity of pig-in-a-poke separation.
"We are offering the guarantee of a constitutional future for Scotland which corresponds with what the great majority of Scots want."
Mr Darling said there is a "remarkable degree of unanimity" about the proposals for further devolution.
"For the first time ever you now have the three political parties that support the No campaign all saying that they want to see further powers, it isn't just tax, it's welfare and other measures as well," he stated.
"The political parties are talking to each other and if Scotland votes in the referendum to stay in the UK, they will then be able to bring forward proposals and they will want to talk to each other, because there is an awful lot of common ground."
But Mr Darling said the referendum campaign must not be seen as "an auction of patriotism, in which Nationalists are the highest bidders".
He stressed that the "emotional argument" for the union "isn't about waving flags, it's about the emotion people feel for their country".
He said the remaining 100 days would "decide much about our future and, more importantly, our children's future".
The former chancellor argued: "Whatever our problems and challenges in Scotland, breaking up our small island into separate states is not the answer.
"The differences which exist within our society are not defined by a border that separates Scotland and England."
Mr Darling said it is "because we want the best for Scotland that we reject the option of dividing our small island into separate states".
By staying in the UK he said Scots could have the "best of both worlds", with the "security of a big state and the diversity of a small nation".
He described that as being "a combination that works and which should not be given up lightly".
He insisted voting No is "not a negative when the proposition you are rejecting is one that would damage the country you love".
Mr Darling continued: "There is nothing more negative than saying you want to break up your country, divide yourself from your neighbours - work alone, not work together."
He added: "The choice is to unite around a vision of a better Scotland, or to divide. To work together for our practical benefit or to divide out of dogma.
"For me the Saltire inspires. For the Nationalists the Saltire blinds. And it blinds them from what is in Scotland's best interests."