The sentiment has been echoed by a Tory backbench MP, who complained that the UK Government had a tendency to be "overly-negative" in the referendum campaign.
Earlier this week, Charles Kennedy, the former Liberal Democrat MP, made a similar point, saying: "We should be more positive in terms of the way in which those of us on the Better Together side are putting over our key messages."
During the past few months, the UK Government believes that thanks to its series of analysis papers and through interventions by key figures such as Mark Carney, the Governor of the Bank of England, BP boss Bob Dudley and Standard Life's David Nish, that it has been setting the political weather.
In particular, it feels the rejection of a currency union by Chancellor George Osborne, supported by Ed Balls for Labour and Danny Alexander for the Liberal Democrats, could be the seminal moment of the whole campaign.
The approach of continuous attack has been dubbed "the Dambusters' strategy" within Whitehall.
But it is now recognised by Coalition figures that the charge by Alex Salmond of a bullying Westminster Establishment is gaining resonance with Scottish voters and that the emphasis needs to be more pro-Union than anti-independence.
"We've been very successful in putting Salmond on the spot on a number of issues. There has been wave after wave of attacks, which he just can't keep sustaining," said the Coalition insider.
"But there is also a feeling here that there has been too much emphasis on the negative and not enough on the positive; this needs to change. There needs to be an injection of love," he added.
There have already been 12 Whitehall analysis papers with more are expected before the summer, including ones on heritage, debt and welfare. The expectation now is these will be slanted more towards the benefits of the Union.
In response, Angus Robertson, the SNP leader at Westminster, said so-called Project Fear had been "grindingly negative and downbeat about Scotland".
"They seem incapable of communicating anything positive and don't even have a common devolution plan."
The Coalition source's intervention comes as Guto Bebb, the Conservative MP for Aberconwy, also raised concerns the Coalition's approach had been too negative.
He said: "The UK Government has had a tendency to present overly-negative messages in the context of this debate."
He referred to the Coalition's rejection of a currency union, noting how, following a Yes vote, it would be "an empty step for the rest of the UK to decide that Scotland should not be allowed to use the pound because an independent Scotland, should one exist, would be the second largest market for rUK exports".
Mr Bebb added that while he did not think Scots would vote Yes, some of the Coalition's arguments "border on being excessive".
Last night, Stewart Hosie, the Nationalists' Treasury spokesman, seized on the comments, saying: "This is a major embarrassment to Project Fear; even the Tories' own MPs know that George Osborne's currency threats are nothing but bluff and bluster. But it's not working as polls move towards Yes."