'I'm pretty confident this is going to be the closest I've ever come to winning an election. Even 40% for the Green party would be something to be excited about'

It is difficult to imagine the First Minister getting "excited" by Yes polling 40% next month.

Harvie believes a Yes vote is still "very possible", but defeat would not crush him. "I won't think it's the end of the world," he says, sitting in a cafe beside Glasgow Central station. "I'll probably give it a bit of a sigh."

Harvie is technically joint convener of the Scottish Greens but is effectively its figurehead and driving force.

A three-term MSP, he is not yet a household name but has a rising profile and a good reputation among fellow politicians.

However, the figures don't lie and the Greens' electoral progress has stalled at Scottish Parliament level. At the 2011 Holyrood poll, the Greens won 4.38% of the vote, up a tiny 0.8% from the first Parliament election in 1999. Why has more progress not been made?

"The creation of the Scottish Parliament was a huge opportunity for us, but it was also a distraction," he says. "What you need to do to make progress is not be effective parliamentarians.

"What you need to do to make progress is build your branches, build your campaigning capacity and the skills of your activists."

He says the Greens failed to do this midway through the 2003-07 Holyrood session: "We were getting recognised as making a serious contribution to the debate, taking the job seriously.

"And we didn't then turn round and say, 'credibility built, let's face outside of the chamber'."

Vigorous internal debates also take place on how the party should explain its policies to voters.

It is said that former Green MSP Mark Ballard and Edinburgh University rector Peter McColl tend to favour more aggressive language while Harvie supports a more inclusive approach.

Harvie says: "There are differences of presentation more than policy. The party sometimes has a bad habit of making detailed policy on technical aspects of environmental management, instead of thinking about what a political party needs to do to connect with voters."

The Greens initially withheld support for Yes Scotland but eventually signed up and Harvie now sits on the advisory board.

However, after he left the organisation earlier this year, Green stalwart Stan Blackley said Yes Scotland had been "ill-devised and dysfunctional from the start, and continues to underwhelm due to a lack of leadership, strategy and resource".

Is that accurate? "It's certainly not the way that I would have put it," Harvie replies. "I think everybody knows we've had our problems with the way Yes Scotland was created at the start. The cross-party and beyond-party bridge building, relationship building, should have happened first. Instead, the SNP set up an organisation … but that's two years ago now."

Harvie tries to be constructive, but it is clear the SNP and Greens maintain an uneasy relationship. Asked whether he likes the First Minister, he says: "I'm not a personal fan."

The SNP leadership also favours currency union, the monarchy and corporation tax cuts, whereas the Greens support a new currency, a republic and big business paying more into the public coffers.

Harvie also believes the SNP's strategy of reassuring voters has sometimes focused on the wrong institutions.

For instance, he is not impressed with the Nationalist plan to replace the BBC: "Far more people would be concerned about losing the BBC than are concerned about whether we are in Nato.

"The BBC is a bigger part of most people's daily lives."

"If you are committed to this reassurance strategy … surely you would be focusing on how the BBC could be a better, stronger organisation that's better for Scotland as well as better for the rest of the UK."

His party's position on a post-independence constitution is also distinctive, with Harvie believing common ownership of land and renewable energy infrastructure should be enshrined.

He also says a public vote may have to be called to ratify a constitution: "I suspect there would be a strong case for putting it to a referendum to adopt the permanent constitution, once it's been drafted."

Harvie is on the non-partisan wing of the Yes side, but he does have red lines. Asked whether he would share a platform with Wings over Scotland founder Stuart Campbell, he said: "No … because I think some of his contributions to the debate have crossed a line."

How about Tommy Sheridan? "I think there are lots of people who have got a major contribution to make to this campaign who don't have convictions for perjury."

Sources on the Yes and No sides believe Harvie is having a good referendum.

His party, regardless of the result next month, seems well-placed to add to its tally of MSPs in 2016.