The Prime Minister warned the Better Together campaign that it had work to do to make an “emotional and historic” case for the Union in the run-up to September’s referendum, suggesting unionists have yet to make a successful emotional case for Scotland staying in the United Kingdom.
The Conservative leader said he believed the anti-independence Better Together campaign was already winning the economic arguments for Scotland remaining within the UK.
But with less than nine months to go to the crucial vote, Mr Cameron said the No campaign now needed to “win some of the arguments of the heart”.
The Prime Minister’s intervention came after reports that senior UK Tories believed former Labour chancellor Alistair Darling’s leadership of the No campaign was “comatose” and “useless”.
Pro-independence campaigners last night claimed Mr Cameron’s remarks showed the No campaign had failed to “convince people that Scotland would have a brighter future as part of the UK”.
First Minister Alex Salmond also criticsed Mr Cameron’s comments, highlighting his refusal to take part in a televised debate on independence and describing his position as a mixture of “arrogance and fear”.
In an interview, the Prime Minister said the UK offered security in a “diverse, dangerous world” as he promised he would “work very hard to play my part” in the cross-party No campaign.
Mr Cameron warned unionists against complacency in the face of opinion polls showing the pro-independence Yes campaign was trailing, and indicated that the No campaign needed to do more to excite voters and make Scots aware of the benefits of remaining as part of the Union.
He said: “We’ve just got to keep pushing both the arguments of the head and those of the heart. I think the arguments of the head we’ve been winning very strongly. We now need to win some of the arguments of the heart.
“The UK is not something to want to belong to simply for economic reasons, but for emotional and historic reasons.”
But his comments were criticised by Mr Salmond, who said yesterday that the Prime Minister had been guilty of arrogance.
The First Minister said: “David Cameron’s attitude is the same mix of arrogance and fear that saw the Tories seek invisible cuts to Scotland’s budget in the 1980s and plan £4 billion of cuts for the future.
“Arrogance because his government wants to dictate the terms of the debate, but refuses to take part in a public debate, and fear because he knows, as people across Scotland do, that he represents a government Scotland did not elect.”
Meanwhile, Mr Cameron hinted that he would be increasing his involvement in the UK government’s campaign against independence, which has largely been led by Liberal Democrat Scottish Secretary Alistair Carmichael and his predecessor, Michael Moore.
The Prime Minister said: “In a diverse, dangerous world, the security of the United Kingdom, the ability to be part of something that could be a great success story, just as it has been in the past – we need to win those arguments.
“Obviously, I’m very concerned that we win this referendum. I think the argument has been going the way of the United Kingdom, but it will not be won until every last vote is counted. I’ll work very hard to play my part.”
Mr Cameron also reaffirmed his opposition to a televised debate on independence with Mr Salmond. He said: “This is not a debate between me and him. The debate should be between people in Scotland who want to stay and people in Scotland who want to go.”
Mr Cameron agreed that the referendum decision would affect everyone in the UK, but insisted the debate should take place in Scotland.
He said: “We debate these things in parliament and in the media.
“But the key question – does Scotland stay in the UK or does Scotland leave the UK? – that is for Scots to decide.
“I don’t have a vote in this. It is for Scots to decide and that is where the debate should take place.”
But Mr Salmond said: “The Prime Minister’s position is increasingly ridiculous. He cannot on the one hand tell the BBC that the referendum debate will take place in the media and then simply refuse to debate.
“A majority of people, not just in Scotland but in the rest of the UK, think it is right that the Prime Minister takes part in a debate and I would suggest David Cameron listens to them.
“He can only hide from this for so long. Eventually he is going to be dragged into the television studio and have the democratic responsibility of an open, free debate.”
SNP MSP John Wilson called Mr Cameron’s intervention a “desperate attempt” to liven up the Better Together campaign.
Mr Wilson said: “The No campaign has failed to touch the hearts of the Scottish electorate and to convince them that they would be better off by voting against independence.
“The policies now being inflicted on Scots such as the Bedroom Tax are making people realise that the only way they will get a fairer society is by voting for independence.
“David Cameron’s desperate attempt to talk about the hearts of Scottish voters highlights the failure of the UK government and of Better Together to genuinely convince people that Scotland would have a brighter future as part of the UK.”
Labour MSP Richard Baker, a director of Better Together, insisted that the campaign was successfully making a patriotic and economic case to maintain the UK.
Mr Baker said: “Better Together has done well so far, but we are not at all complacent and want to win the argument about people being proud to be Scottish and British as well as the economic argument.
“There is a strong emotional element to the case for Scotland’s place in the UK and we’ll continue to address this and the economic case throughout the campaign.”