If I told you one party leader in particular was having a gas of a campaign, you'd probably assume I was talking about Nicola Sturgeon.

Hurling around on zip wires, diving down soft play slides, challenging journalists to go-karting contests -- only someone comfortably in the lead and without a care in the world could be having so much fun.

In fact, the man behind these off-beat photo-calls is Willie Rennie, leader of the Scottish Liberal Democrats. In a deathly dull election, where everyone already knows the outcome, Rennie has brought much-needed levity to proceedings.

At times, it's seemed more like an extended stag do than an election campaign, but Rennie says that's the point.

He tells me: "I've been travelling around Scotland doing lots of things that I quite enjoy, so zip wires and play parks and meeting kids. It's been good. To be honest, it's me. It's what I like doing. I like to have a bit of fun; there's no point in taking life too seriously -- whilst mixing it with some pretty heavy issues, like mental health and drugs and prostitution.

"Those are not light issues so you should be able to mix the two and show people that politics is not as boring as many think it is. It's interesting seeing the response from people on Facebook and social media. When you do something that just piques their interest, then you can open the door to discuss other things.

"I'm a politician who wants to make big social changes but it doesn't mean you have to go around with a dripping face all the time."

Rennie is the happy warrior of the Liberal Democrats. No Lib Dem figure, in Scotland or nationally, comes close to his optimistic outlook on political life. You have to keep it upbeat when you're running the Lib Dems in Scotland, where the party was punished heavily for its coalition with the Conservatives. The surge in nationalist sentiment following the 2014 referendum and the SNP's continuing march to the centre on social and economic policy has left little in the way of breathing space for the Lib Dems.

But if the polls are anything to go by, the party could increase its seat tally -- if only by one or two -- on Thursday night. Cynics might dismiss that eventuality as a quirk of Holyrood's electoral system but it's a bit churlish to overlook the impact of Rennie's buoyancy and affable dogging of the Scottish Government on its authoritarian policies.

Still, he refuses to talk targets, insisting: "It's all about direction of travel and we're going to grow in this election. We've turned the corner; we're back to our best. It's not just the slogan for the country -- it's the slogan for us as well.

"We're drawing inspiration from people like Charles Kennedy, whereby we're back to that social liberal -- combined with economic competence -- but that social conscience and progressive politics. I think that means that we're going to grow this time and, for me, I've got no limits on what I want to achieve so I'm not going to put any limits on that.

"What I do know is that once a momentum starts, you don't know where it's going to stop. What we're getting in this election is people who were Liberal Democrat before, and maybe have gone off to the SNP because they were annoyed with the coalition or whatever other reason -- they're now coming back. I'm not saying it's a stampede but when you get movement, you don't know when it's going to stop.

"So the momentum's with us and the momentum is now against the SNP."

God love him, he believes it too. While it might be the case that Thursday will mark the peak of Natmania in Scotland -- I wouldn't count on it -- Nicola Sturgeon's party has a long way to fall.

And just as long, if not longer, will be the Lib Dems' efforts to rehabilitate themselves after the 2010-15 coalition government with the Conservatives. Admirably, Rennie has never ratted on this subject, loudly defending Nick Clegg's decision even when it would have profited him to join in the feel-good outrage.

In the long term, he assures me, history will look favourably on the party for putting the national interest ahead of popularity.

He insists: "When people see clearly what we were prepared to do, and I think the history books will serve us well, they will see that at a time of great national crisis -- and it was -- when the Tories could have been running the country by themselves, we were there making that case. We stepped up rather than skulking away.

"We could have come up with excuses. We could have skulked into the background and said we didn't get this, that, and the other. But we weren't going to do that. It was the right thing to do at that time and I think people increasingly see that."

Rennie's pitch to the voters is standard centre-left fare but in a country where left versus right has been replaced by nationalist versus unionist, it sounds fresh and original. And while he opposes a second referendum on independence, he has not made it the central theme of his campaign, as is the case with Ruth Davidson and the Scottish Tories.

As he puts it: "A penny on income tax, guaranteeing our civil liberties, protecting the environment and also giving a big boost to mental health services. Four big, colossal liberal values and that contrasts with the miserable prospects being offered by the Conservatives in this election, which is a return to the arguments of the referendum that we've already resolved.

"Ruth is saying the SNP just want to open up the divisions of the referendum. That's exactly what she's doing -- exactly what she's doing. Let's put the divisions behind us, let's bring the country together, let's focus on the optimistic agenda for the future in the way I've just described, and that contrast is quite clear."

Where he echoes Davidson is in his belief that Nicola Sturgeon should be held to her promise that the referendum was a "once in a generation" opportunity. Fair enough but surely if public opinion swings behind Sturgeon on this, she has every right to have another go?

"So she's just going to beat us until we submit?" he demands, bluntly.

"I think that's the idea," I suggest.

"What kind of respect is that for democracy?"

It's testament to the scarring effect of constitutional division that we no longer have a shared concept of popular sovereignty in Scotland. The fact is that Sturgeon has the means, the motive, and if the polls tilt her way for long enough, the opportunity to reopen the independence question.

Yes, she is going to beat unionists until they submit but when she does, the club will have been handed to her by the voters. The SNP has not respected the result of the referendum but the unionist parties refuse to recognise the right of the voters to change their minds. Until they make them up once and for all, Scottish politics will remain a running battle between between flags and competing identities.

One advantage to getting a final answer on the constitution would be the chance to pursue critical but unsexy matters, such as our mental health crisis. The Scottish Lib Dems want to double the mental health budget for children and young people; train mental health professionals to work with GPs, police and in prisons; and recruit more GPs by tripling the primary care fund.

It's part of an observable shift in attitudes towards mental wellbeing in recent years.

Rennie is welcoming of the change in outlook: "It's the kind of issue that people, in the past, would shuffle about in their seats. They were embarrassed about it. Now, you get nods around the room. That shows that, by providing a bit of political leadership, you can change the culture and just speaking about those things often does enough to move things forward. But we do need money, we do need NHS spending on mental health to at least keep pace with the rest of the spending, and grow in our view, but it shouldn't fall behind as it currently has been.

"We need to get a mental health strategy back. We had one of the best in the w orld. It was SAMH who were saying countries across the world were looking to our strategy. We haven't got one anymore, which is really embarrassing. It just shows up where the SNP let things slip in the last five years because of the constitution.

"It's really important for me to stand up for people who are dispossessed -- the voiceless -- that's why I campaign for it. Probably everyone secretly knows someone who has suffered from this and the example I used on the STV programme was about a constituent whose daughter was self-harming and that just breaks your heart.

"Another one who came to me, a young adult, had been waiting eight months just to get some counselling. Now, if she's got that earlier it could have staved off more problems. I hear it all the time but the more it's exposed the more we can get it raised."

I wonder if Scotland's problem is that we are at root an innately illiberal country.

"The SNP are illiberal," he corrects me.

But, I press, they enjoy a great deal of support, including for many of their more authoritarian policies. These include prosecuting Irish republicans for singing political songs; an abortive attempt to abolish corroboration; centralising Scotland's police forces then allowing them to arm officers routinely and stop-and-search an alarming number of Scots; setting a minimum price on alcohol; extending the ID database; and banning GM crops without scientific consultation. Their Named Person scheme -- which the Lib Dems "cautiously" support -- is the only power grab that has managed to stir a backlash.

Is Scotland fundamentally an authoritarian country?

He ponders this a while. "I don't think so... It's what you pander to... Political leaders are there in part to lead, not just to follow the lowest basic instinct that exists. I happen to believe that lower, basic instinct does exist in Scotland but it's a minority.

"Most of the people I speak to are open, tolerant, trusting, liberal-minded, live-and-let-live, internationalist, outward-looking -- most of them are that. But if the SNP keep on pandering to that minority view, they're going to feed it and I don't think that's a helpful thing to do.

"The SNP claimed to be liberal before -- that's all gone, clearly gone. So I don't think it's a majority view. I think there is a view that supports those things, that looks to fear rather than to hope, but we're a hopeful party and that's why we need to stand up very clearly against these things.

"Look at the support we've had over Police Scotland. Huge support; people disgusted with what happened. People really astonished that the stop-and-search levels were so high. The central belief that Scotland is illiberal, I don't accept. Equal marriage is the obvious example where it's not that. The SNP need to show more of that leadership rather than the illiberal leadership that they provide."

Liberalism is on the back foot worldwide, as nationalism and populism soar in the sclerotic recovery from the global financial crisis. People are angry and to the angry nuance sounds evasive and underhand. Say what you like about flags and slogans, there's nothing complicated about them. They get the message across.

There is still an alternative message, one that offers moderation, open-mindedness and critical inquiry. Willie Rennie can only hope it is a message some Scots are still willing to listen to.

Stephen Daisley is STV's digital politics and comment editor. You can contact him at stephen.daisley@stv.tv.