Party leader interviews: Nicola Sturgeon

If she can translate that success into seats, the SNP could wield unprecedented influence at Westminster as it tries to end austerity, secure more powers for Holyrood and scrap Trident.

It's a far cry from general elections past. In its 81 years, the SNP has only once made it into double figures, when it won 11 MPs on a 30% share of the vote in 1974.

Since then, it has had a numerically irrelevant handful of MPs. Not any more.

All the polls suggest that on Thursday the SNP will a majority of seats in Scotland and become the third largest party in the Commons.

Regardless of who is Prime Minister, that change in power dynamics means we are about to enter a new and unpredictable era. As Lord Gus O'Donnell, the former cabinet secretary, has pointed out, it also means "extra tensions for the union".

Sturgeon is coy about putting a number on the scale of the win. Like all SNP veterans, she's seen the party squeezed between Labour and the Tories in previous elections.

So she reckons it's "highly unlikely" the party will take all 59 Scottish seats, as one poll predicted. But she doesn't entirely discount it, and with so little time for the polls to shift, you sense her ballpark figure wouldn't be much off it.

"Potentially it's a historic election for the whole UK," she says, snatching a rest from street campaigning at the SNP's election HQ in Edinburgh.

"It's an opportunity to break the two-party binary system, to have an age of more representative politics at Westminster. If people in England were satisfied with one or other of the big parties, they would have a healthy lead in the polls. The deadlock suggests there's an appetite for change there as well as here. This has got the potential to be a watershed election, not just for Scottish politics but UK politics."

Sturgeon credits the referendum for triggering a "fundamental shift" in public opinion.

"Whether people voted yes or no, the referendum gave people a sense of empowerment," she says. "The future of the country was down to us. Suddenly we didn't feel powerless in the face of big, remote Westminster. Even people who voted No liked that. Since then there's been a feeling of not wanting to give up that sense of empowerment. We've chosen to stay part of the Westminster system, but we don't want to be a forgotten, sidelined part of it.

"So there's a clear sense that voting SNP makes our voice louder. It means that Scotland's interests are further up the agenda. You've got MPs there that fight Scotland's corner. I think it's that simple."

You're about to dethrone Labour. Why didn't they see it coming? Denial, she says.

"The SNP became a minority government in 2007, then a majority one in 2011. But Labour viewed what was happening as some kind of aberration. They felt the problem wasn't theirs: they didn't have to change, the Scottish people had just gone down this wrong road and if they waited long enough they would find their way back.

"That's the fundamental mistake. Instead of trying to say, 'Maybe we're the problem, maybe we've got to change' they assumed the people got it wrong."

With Sturgeon rated the most popular UK party leader, the SNP is understandably giving her a massive presidential-style push, putting her face on leaflets, posters, even a helicopter. More modest than her predecessor, it's not always comfortable for her.

"I passed a horrifyingly large, lit up billboard last night," she shudders. "I didn't like that very much at all."

There's another flipside to fame. It has made her the focus for enemy attacks, such as David Cameron's warning of a "nightmare" alliance between her and Ed Miliband.

How do you feel about being a recruiting sergeant for the Tories?

"I don't believe that's true," she insists. "I absolutely don't believe that's true. There's few things I want more in life politically than seeing the back of the Tory government."

Sturgeon has also been attacked over the SNP's choice of candidates, many of whom can be seen going merrily off-script on YouTube.

Do you take responsibility for the candidate selection? "I take responsibility for everything that happens in the SNP as leader."

So if some of your candidates blow up after the election, it's down to you? "Ultimately it's one of the responsibilities of leadership. Ultimately the buck stops with me, as it stopped with Alex for the last 10 years. That's part of being a leader."

The SNP's high hopes turn on Miliband being Prime Minister in a hung parliament in which he is sustained by either explicit or tacit SNP support.

Miliband ruled out a deal on Thursday, but the SNP think that still allows for low level "dealings" that could see the parties work on a vote-by-vote basis.

"If there is an anti-Tory majority, the only thing that would stop him being Prime Minister is him, if he opted not to be," Sturgeon says. "We would work with him to make him Prime Minister. We'd then use our influence to try and make sure he did good things."

But while she's keen to use Miliband to crowbar David Cameron out of Downing Street, she's less emphatic about whether the Labour leader deserves to be there instead. There's clearly more calculation than admiration involved.

Is Miliband Prime Ministerial material? "That's for voters to judge," she shrugs.

But you're offering to put him into the job. "He's standing for Prime Minister. The only reason I hesitate is that I don't know Ed Miliband personally. But where there's an anti-Tory majority, I would rather he was Prime Minister than David Cameron."

Even though he might turn out to be a B-lister, a bit of a diddy, you'd be prepared to give him the job? "That's where a large number of SNP MPs can play a big role."

But they can't improve his personality or give him more brains. "Politics is about policies and decisions and all the rest of it, and that's where we have the opportunity, if people vote for us, to exercise a lot of influence. Our job, if the numbers allow us to keep the Tories out, is make sure they're replaced by something better."

Could success at Westminster blunt the case for independence? If you succeed in your goal of 'fixing' the institution, doesn't that weaken the case for leaving it?

"Time will tell. Right now my primary interest is folk suffering because of Tory welfare cut, who are denied opportunity because the Westminster system doesn't work.

"My immediate objective and priority in politics is to try and make things better for all of us who live in Scotland right now. If I was to judge everything all the time on looking two or three steps ahead - what does this mean for the SNP's ultimate goal? - we'd very quickly lose all the trust that people in Scotland have in us. It would be totally counterproductive. The most important thing is to do the best you can for the country as you see it in the here and now. People will judge you on that."

You've said repeatedly that this election is not about independence or a second referendum. SNP canvassers use a standard script on the doorstep. What's the final question they ask? "We ask people's views on independence."

Why? "Partly because it's part of our computer system," she says unconvincingly.

Doesn't it show, despite what you say, that there is no time-out from independence, that you're always thinking about it? "In any canvassing I've done in this election, I actually haven't been asking that question. It's not compulsory. It's there on our script. Many people will ask it, some people won't. We're a political party. We gather data on people's political views, but whatever our canvassing script might say, whatever our Activate [computer] system records, this election is not about independence."

Yet you're asking people 'Should Scotland be an independent country?'

"Well, it would be kind of odd for the party that supports independence not to be interested or in whether or not people support it. The question of independence will only be settled in a referendum."

When to hold that referendum is one of the biggest conundrums facing Sturgeon.

If she includes it in the SNP manifesto for the next Holyrood election, it exposes her to the charge of ignoring last year's result and could inspire a tactical anti-SNP vote.

But if she excludes it, the SNP's gung-ho new membership could kick off and the party could be damaged by a messy public split. Small wonder her opponents are taunting her over the near-unplayable issue. Sturgeon improbably claims not to have thought about that bit of the manifesto yet.

If SNP national council or conference [the party's policy making bodies] vote for a referendum to be in the 2016 manifesto and you disagree, could you overrule that?

"Ultimately as party leader... any party leader will sign off the manifesto," she says.

So you could overrule it? "I sign off the manifesto."

SNP members didn't join for that. "The SNP knows no matter how much we want independence - and we do, all of us want it very much - that's not enough. We have to be able to persuade a majority of people. So I will sign off the manifesto. But whether a proposal was in the SNP manifesto or not, that in and of itself does not deliver a referendum. People then have to vote for that manifesto. So the question of whether there's another referendum or not is determined by voters, not by the SNP."

A couple of quickies to finish. When did you last meet Rupert Murdoch? "I've never met Rupert Murdoch," she says, genuinely startled, suggesting her predecessor did the groundwork for last week's endorsement by the Scottish Sun. "His company is an employer in Scotland, and I'm sure at some point I will meet him." No doubt.

What do you think of Jim Murphy? "I have no personal animus towards Jim Murphy."

Do you rate him? "I'm asking people to rate and vote for the SNP. It's up to Jim to argue his own case. I'll leave it to other people to draw their own conclusions."

There's a little smile. She knows voters already have.

Labour's Jim Murphy was the only Scottish party leader to refuse an interview with the Sunday Herald