Nicola Sturgeon was the SNP's Alex Ferguson - so will her successor be their David Moyes?

Nicola Sturgeon’s decision to step down as the longest-serving first minister and leading force in trying to deliver an independent Scotland is an inflection point, not just for Scotland but the whole of the UK.

A polarising politician, she was nevertheless an exceptional communicator and campaigner who has dominated Scottish politics for a decade.

As Ms Sturgeon said herself in her resignation media conference, she fought eight elections as first minister and won them all.

She has been Scottish nationalism's leading light and campaigner-in-chief and as the SNP now search for a new leader, unionists and opposition parties scent an opportunity: could this be their moment to loosen the iron grip the SNP has had on Scottish politics since Ms Sturgeon took charge?

And for the SNP, can it manage succession from such a big name to a relatively unknown one while being able to repeat her electoral success?

Think of another Scot, Sir Alex Ferguson, who delivered sustained success at Manchester United for 26 years, winning 38 trophies including 13 Premier League titles.

The football club haven't won the league since Fergie retired a decade ago.

None of the three candidates now vying for the SNP leadership would want to be compared to his successor, David Moyes, who was sacked after less than a year.

But there are inevitable questions about whether any of the three leadership candidates can even begin to slide into Ms Sturgeon's shoes.

This is an inflection point, then, that could define the direction of the SNP and the future of the Union for years to come.

Because as this power vacuum opens up in the party, the divisions that were becoming more apparent in the final months of Ms Sturgeon's leadership are now more visible and deepening as she exits office.

This leadership race has sparked discussion and disagreement over how the SNP should evolve and how - if even - the party can achieve its core aim of independence.

Forbes, Yousaf or Regan?

This week's first TV debate, hosted by STV, exposed just how deep those divisions are. A party that used to be known for its discipline put on a very different display.

In the cross-examination section of the debate, Finance Secretary Kate Forbes turned on the Irn-Bru swigging frontrunner Humza Yousaf and his track record in government in a demolition job that raised questions about the SNP's record in government, as well as Mr Yousaf himself.

"You were a transport minister and the trains were never on time, when you were justice secretary the police were stretched to breaking point, and now as health minister we've got record high waiting times," she said. "What makes you think you can do a better job as first minister?"

Mr Yousaf was made a minister by Alex Salmond at just 27 years old and went on to hold big briefs in Ms Sturgeon's cabinet.

He is the 'continuity candidate' in this race, publicly backed this week by the SNP's former Westminster leader Angus Robertson, but the blunt accusation from his leadership rival is that Mr Yousaf's ambition to be Scotland's next leader isn't matched by his competence or ability to do the job.

As for Ms Forbes, she has positioned herself as the change candidate in this election, but she faces questions of her own over exactly what kind of new direction she would set.

Traditional values in a progressive party?

While the SNP defines itself as a socially progressive party, Ms Forbes is a devout Christian with very traditional views on family life, which raises obvious questions around whether her values and that of the party she hopes to lead are aligned.

This tension was exposed in the first week of this race when the 32-year-old Highland MSP criticised her party's legislation on reforming gender laws, confirmed she would have voted against gay marriage had she been an MSP when that law was passed in 2014, and suggested to Sky News that children should not be born out of wedlock.

These remarks led some MSPs who had previously backed her to withdraw support and even her own campaign manager appeared to have a public wobble.

It has put Ms Forbes on the backfoot as her opponents try to press it to their advantage:

"Forget persuading No voters, you can't even keep Yes voters onside," was Mr Yousaf's response.

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There are also questions now about whether Ms Forbes can keep political allies on side: regarded as a competent minister with sharp intellect, if she was to win the Green Party would probably walk out of its coalition with the SNP, while some pro-independence LGBTQ+ activists have threatened to quit the Yes movement.

The third candidate in the race is Ash Regan, Edinburgh Eastern MSP, who hit the headlines last year when she resigned from Nicola Sturgeon's government over its gender reforms. The outside candidate, Ms Regan is trying to connect with SNP members who want more focus on independence.

One of her proposals is to build a "readiness thermometer" to measure Scotland's progress towards independence with "a dial on it that moves".

There is little expectation that Ms Regan will win the race, but she could still have a big impact on the outcome.

SNP members have two votes in this election, and it is, perhaps, significant that her most prominent supporter, Joanna Cherry MP, has made it clear that Kate Forbes is her second choice.

Could Ms Regan's role in this contest, and that of her supporters, be to help put a block on the SNP establishment choice?

End of an era

Nicola Sturgeon's decision to stand down is the end of an era, and the big question for whoever wins this race is whether they can even approach the success Ms Sturgeon commanded at the ballot box when the new era of leadership begins.

With the SNP struggling to find a route through to achieve its core goal of Scottish independence and a Scottish government under real scrutiny over policy and performance, the landscape is getting rockier just as a less seasoned politician prepares to take the reins.

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With a general election a year to eighteen months away, the public will soon get an opportunity to give their verdict on the SNP members' choice and what happens in Scotland will affect the outcome of the next general election for the whole of the UK.

Will the end of the Sturgeon era precipitate a beginning of the SNP's decline from such a high electoral watermark?

That's still to play out, but how these three candidates present themselves and their visions to the public, will be an important part of that story - and our Sky News Debate will be an opportunity for us to put all three candidates to the test and for you to decide who comes out on top.