Other leading figures in the pro-Union party backed his stance, with former leader Tavish Scott attacking ex-chancellor Alistair Darling over his performance at the helm of the No campaign and calling on Labour to roll out its “former stars” to mount a fightback.
Deputy Prime Minister Nick Clegg said that the prospect of staying in the UK must be as “thrilling” for voters as independence, as the Liberal Democrats gathered in Aberdeen yesterday for their Scottish conference.
Yesterday, one SNP MSP said the Lib Dem criticism showed splits within Better Together.
It came as officials behind the Better Together campaign held a board meeting in Glasgowdenying claims that it was a “crisis” response to a narrowing of the opinion polls ahead of September’s referendum.
The official pro-Union campaign has pledged to run a major advertising campaign, including a series of special adverts to be shown in Scotland’s cinemas, in the coming months which they say will emphasise the positive case for a No vote.
Mr Rennie will make his keynote speech to the party conference today but warned in an interview yesterday that a Yes vote was increasingly likely.
“I think there’s a possibility Scotland could be independent in September. I think there’s a distinct possibility,” he said. “That’s why we need to focus really hard on what that means.”
He said the pro-UK campaign must now adopt an approach which “emphasises the positives” of the Union.
Mr Rennie said: “I’m hoping Better Together will embrace that sunshine strategy over the coming months. People need to know there’s something great about the United Kingdom. We need to remind them what that is.”
He added that attacks on the case for independence would continue: “We have a right to ask difficult questions without being accused of being negative, but let’s emphasis the great things about the UK, too.”
His remarks came as a senior Scottish Liberal Democrat announced he planned to vote for independence in September.
Former party treasurer Denis Robertson Sullivan said he was voting Yes because he wanted Scotland to stand on its own feet, following a similar announcement by former party chiefexecutive Andy Myles.
Speaking at the conference, Tavish Scott called on Labour to use “former stars” like Gordon Brown, ex-home secretary Lord Reid and ex-Nato secretary-general Lord Robertson to make the case for the Union.
“I don’t think Alistair Darling is pulling that whole swathe of people back into definitelyvoting for the Union,” he said.“What Alistair does is make an intellectual case, which he does very well. But it’s a different sub-set of the electorate.
“What they really have to recognise here is the SNP has segmented the electorate into categories where they know they can attract people if they get the right kind of message.
“I don’t think Better Together has done that yet.”
He added: “Our voters will vote No, Tory voters will vote No. The voters who matter here more than anyone else are this group in the middle who are Labour-voting. They’re the ones you need to get a hold of.
“I think the key part of the electorate, that is potentially really motivated to vote in this referendum, are people who really do despair and can be won by an explicit message of just simply hope – and I give credit to the SNP for recognising that.
“I think the way to counter that, the strong counter to that, is personality.
“I always felt at some stage this issue would start moving, and what was Better Together’s response? My concern at the moment is I’m not clear what Better Together’s response is.”
Chief Secretary to the Treasury Danny Alexander last night rejected claims that Better Together was being too negative.
But the Lib Dem MP forInverness, Nairn, Badenoch and Strathspey said it must “step up” its campaign.
He said: “I think Alistair Darling is doing a very good job leading the campaign and particularly putting across the economic arguments. He’s very powerful and very convincing.”
In his address to the Scottish conference, Lib Dem leader Nick Clegg stressed the importance of setting out the positive case for the Union.
“Just as it is right that we must explain the risks and consequences of Scotland voting to leave the Union, we must also set out the opportunities of voting to stay,” he said.
Nationalists last night said the admission from the pro-Union side that Scots could back independence illustrated how “negative” tactics had backfired.
SNP MSP Christian Allard said: “These comments go right to the heart of the crisis that is engulfing Alistair Darling and the No camp as cracks turn to splits. With the momentum building behind Yes and support for No plummeting, Tavish Scott is right to say that the No campaign simply has no response.”