His party will fight the Holyrood elections on a manifesto promising a 1p increase on the basic rate of income tax with the money raised ring-fenced for education.
Tax rises are not normally seen as a vote-winning strategy, but Mr Rennie said: “The situation is so urgent now, the proposed cuts to schools are so drastic that people will now accept it more.”
He said the effect of a tax increase on lower earners was now much reduced since many people had been taken out of tax altogether by higher tax thresholds and no-one earning under £19,000 would pay more.
“It’s a small contribution we’re asking people to make for a big return,” he said. “But even if nobody voted for us on that basis, I would still propose it because it’s the right thing to do.”
Mr Rennie – whose party holds its spring conference at the Assembly Rooms in George Street tomorrow and Saturday – is confident the Lib Dems will do better in the May election than at the last one in 2011, when they were reduced from 16 MSPs to just five.
“We’re looking for growth this year,” he said. “It’s about increasing our numbers to provide even more weight for a strong liberal voice.”
He said the Lib Dems had spoken out on nursery education, college cuts and problems with Police Scotland.
“If we weren’t there a lot of these arguments wouldn’t be getting made,” he added.
Mr Rennie is sceptical about claims the Tories are poised to make some kind of comeback.
“They’re appealing to those who are always Conservative with Conservative ideas. I’m not sure they’re really reaching out beyond their core support.
“A lot of people voted for independence to get away from the Tories. I’m not sure how they solve the problem of independence with more Tories when they created the problem in the first place. The solution to the problem the Tories created is not more Tories.”
And he accused Scottish Tory leader Ruth Davidson and the SNP’s Nicola Sturgeon of not wanting to move on from the constitutional debate.
He said: “It’s pretty clear Ruth and Nicola want to continue the independence debate because it polarises opinion in their favour when the country is crying out for debate on the big issues – education, environment, police and the health service. That’s where the focus should be. Scotland has suffered in the past five years because we have been utterly obsessed by the constitutional debate on independence.”