The Scottish Liberal Democrat leader claimed the SNP’s Growth Commission had been “astonishingly mishandled” and made it easier to argue the Union.
“Nicola sounds increasingly desperate, almost panicky, about what she’s done,” Mr Rennie said, putting the report alongside the First Minister’s abortive push for a second independence referendum.
“She’s obviously tried to bring a bit of realism to the independence message, but the realism now sounds dark and despondent... You’ve got a nationalist camp that’s divided as never before.
“Their criticisms of Brexit could equally be applied to independence. That’s the big opportunity for us”.
The Scottish Lib Dem leader said: "If you look back to post-Brexit, she wasn’t sure footed at that point, where she rushed towards another independnece referendum, ignoring public opinion… she didn’t pick it up for a good six to eight months.
"That was probably the biggest political mistake of her career. I think this could be the next biggest mistake of her career."
He added: “From all different flanks, from Labour’s austerity message, from our European message, from the fundamentalists in their own party, they’re being attacked from every direction now.
"The dynamics have changed fundamentally, and I don’t think a more negative, pessimistic approach is going to help them at all. It’s what we’ve been arguing all along. It’s great to have them admit it at last.”
Mr Rennie joined Lucy Salek, the Lib Dem candidate in the Lewisham East by-election, at local charity ‘Brighter Horizons’, which stepped in to offer day services for adults with learning disabilities after the council shut drop-in centres.
Lewisham voted by a 2-to-1 margin to stay in the EU, and the Lib Dems hope to claim second place in an area where they won 11,000 votes in 2010.
Mr Rennie admitted the party’s national polling was “puzzling”, but said the Lib Dems would draw more support if they could demonstrate “winnability”.
"You’ll see in council by-elections that where we are proven to be relevant, people are coming our way. It is about showing that credibility and that winnability to people," he said.
"They might have previously agreed with us on all the issues we’re campaigning on, but were they really convinced that we could win? Well, now they can be convinced that we can win."