Sturgeon, in office since 2014, unexpectedly announced last week she was resigning as first minister of Scotland’s semi-autonomous government, saying she had become too divisive.
The bookmakers' frontrunner to succeed her and become the next leader of her Scottish National Party (SNP) is Humza Yousaf, a Sturgeon loyalist who faces criticism for his record in government.
Up against him are Kate Forbes, a rising star whose views opposing same-sex marriage have already lost her supporters, and Ash Regan, who quit the government in opposition to proposed changes to gender recognition, as the deadline for nominations closed on Friday.
The campaign so far has been dominated by a debate about views on social issues such as gay marriage, transgender rights and abortion. Sturgeon had faced criticism over her government's plan, which makes it easier for people to change their legal gender.
However, whoever wins will need to restore the Scottish government's reputation for day-to-day competence amid constrained budgets, while members of the SNP want them to chart a route forward towards the ultimate goal of independence.
"What the SNP need is somebody, a rare individual, who combines both of those talents," John Curtice, Britain's best known pollster, told Reuters, adding that the debate over social issues had meant none of the candidates had yet set out their vision for Scotland.
"The big fundamental question of facing the party... is how it's going to acquire majority support in Scotland for independence: That question has not been addressed."
Under Sturgeon, who became leader after an independence bid was defeated in a 2014 referendum, the SNP positioned itself as a progressive, pro-European voice for a Scotland that needed to break away so it could end years of Conservative government from Westminster.
But her attempt to call a new independence referendum without the consent of the UK government was stopped by the UK Supreme Court, while another row when London blocked the gender recognition reform law after its passage through the Scottish parliament revealed as many divisions over the policy within the SNP as it did with Westminster.
When she announced she was leaving, Sturgeon said she could not ask her party to follow her controversial plan to declare the next UK-wide election as a de facto referendum on independence if she was unsure of her own future as leader.
James Mitchell, a professor of public policy at the University of Edinburgh, said Health Secretary Yousaf was "the continuity candidate" who had not been associated with any particular vision of Scotland beyond independence.
He has however been criticised for his handling of a health service still struggling to recover the COVID-19 pandemic.
Yousaf has distanced himself from Sturgeon's independence plan, and has said the party needs to get back to making the case for independence, rather than endlessly debating process.
But his momentum mainly stems from pitfalls suffered by his main rival.
Forbes, the nation's finance minister and a member of the socially conservative Free Church of Scotland, had been the frontrunner until she said she wouldn't have voted for single-sex marriage, which was legalised in Scotland in 2014. Some SNP politicians withdrew their support.
She has caused further controversy with her views on abortion, and by saying having children outside of marriage is wrong and that a trans woman is a biological man, though she has emphasised she will protect the rights of everyone.
One SNP lawmaker said that Forbes' campaign would likely continue to be dominated by her views and distract from other issues, which would be bad for her, the party, and other politicians who are religious but hold progressive social views.
"I can’t see how she can continue. So much damage has been done," the lawmaker said.
However, a poll published on Friday showed Forbes was the most popular candidate in the first survey of the party's supporters since the start of the contest.
Forbes had 28% support, putting her 8% ahead of Yousaf and comfortably ahead of Regan on 7%, while the remainder did not know who to back or preferred another candidate, according to the poll published by Opinion Matters, which surveyed SNP voters rather than party members who will decide the next leader.
Regan has said she would push for independence as soon as possible while looking to use North Sea oil reserves and rejecting the gender recognition law, policies that would strain a governing agreement with the Green Party.
As other policies are debated, Curtice said Sturgeon's pro-European stance was set to endure. Unlike the English and Welsh, most Scots opposed Brexit in the 2016 referendum.
"Sturgeon's framing of the independence debate in the last few years has been very much one in which really getting back into the European Union... is an important vision," he said.