LONDON — Passionate, professional, and a top-notch communicator, Nicola Sturgeon was not just the dominant figure of Scottish politics, but someone whose opinion poll ratings were the envy of fellow politicians during most of her eight years as Scotland’s leader.
So, her sudden and unexpected resignation on Wednesday has raised an awkward question for her pro-independence Scottish National Party: if Ms. Sturgeon, leader of the Scottish Parliament, could not mastermind Scotland’s exit from the United Kingdom, can anyone else?
On Thursday the contest to succeed her looked wide open, though one thing is clear: the new leader will not have the formidable profile of the outgoing one. An opinion poll on potential successors, published not long before Ms. Sturgeon’s resignation, found the leading contender, the finance secretary Kate Forbes, polling at just 7 percent, one point ahead of John Swinney, a former party leader, while 69 percent of respondents picked “don’t know.”
Not only does the her party find itself without an obvious candidate to take over from its charismatic leader as chief of the party and first minister of Scotland, it is also bereft of a blueprint to achieve its overriding objective: the end of its three-centuries-long union with England.
“The problem is there is no obvious strategy for independence,” said James Mitchell, a professor of public policy at Edinburgh University who added that the prospects of achieving it were receding.
“The window is not closed but it’s certainly closing for the moment,” Professor Mitchell said. “That doesn’t mean it won’t reopen in the future, but they need to go away and reflect on what they mean by independence, what are the policies that would be involved and the challenges.”
This is hardly the first setback for the Scottish nationalists, who in 2014 lost a referendum on Scottish independence by 55 percent to 45 percent, a defeat that led Ms. Sturgeon to become party leader following the resignation of her predecessor, Alex Salmond.
Under British rules the consent of the government in London was needed to hold the vote, and David Cameron, then the prime minister, gambled and agreed.
But since then Conservative governments in London have refused demands for a second referendum, and Ms. Sturgeon’s resignation on Wednesday was an implicit recognition that she had failed to find an alternative route.
In several respects the political environment could scarcely be more favorable for an independence vote. The Conservatives, unpopular in Scotland, have won four successive British general election victories — reminding Scots that, with just 8 percent of Britain’s population, their votes can be overridden within the wider United Kingdom with England, Wales and Northern Ireland.
Then there is Brexit, an even bigger symbol of Scottish impotence and disagreement with Britain at large. In the 2016 referendum on European Union membership, 62 percent of Scotland’s voters opted to remain — only to find themselves outvoted.
And last year was one of unbridled chaos in London that did not make being in Britain a more attractive option. Prime Minister Boris Johnson was forced from power after a series of ethics scandals, and his successor, Liz Truss, became the shortest-serving occupant of 10 Downing Street in history.
Still, with London refusing another independence referendum Ms. Sturgeon could not find a way forward, conscious that any unlawful breakaway would make it impossible for an independent Scotland to rejoin the European Union, as she wishes.
Her first plan was to schedule an independence referendum next October, testing the right to do so in the courts. In November she lost that battle when Britain’s Supreme Court ruled that Scotland’s Parliament did not have the right to act unilaterally.
Ms. Sturgeon then suggested that the next British general election, which must be held by January 2025, should be treated as a de facto referendum on independence. But that option, as well as being questionable constitutionally and practically, was also contentious within the Scottish National Party and may now be set aside pending the election of a new leader and first minister.
Frustrated by the lack of progress, some hard-liners favor unilateral action, holding a referendum in defiance of London. Catalan separatists in Spain took that route in 2017, and it led to the imprisonment or exile of some independence movement leaders.
Other Scottish nationalists hope that the next British general election will deliver a close result, leaving the S.N.P. holding the balance of power. Under those circumstances, the party might demand a referendum on independence as the price of its support for a minority government led by the opposition Labour Party.
However, it is far from clear that this would work. For one thing Keir Starmer, Labour’s leader, has ruled out such a deal. For another, Scotland’s hostility to the Conservatives would make it hard for the Scottish nationalists to bring down a minority Labour government without risking a backlash from their supporters (especially if doing so risked prompting a second general election that could lead to a Conservative victory.)
With no clear path to a resolution and public opinion in Scotland split roughly evenly, the debate seems stuck. Historically, such divisive constitutional questions have sometimes taken decades to resolve, said Michael Keating, emeritus professor of politics at Aberdeen University.
“Both sides have lost,” he said. “The unionists have lost the argument but the nationalists haven’t won it.”
So most analysts believe that the fight for independence should focus on building long-term and broader public support for the cause. After all, the union between Scotland and England was entered into voluntarily in 1707, and London would probably be unable to resist a second referendum indefinitely if a clear and sustained majority of Scots appeared in favor.
While Brexit strengthened the political argument for Scottish independence — because Scotland was removed from the European Union against its will — it complicated the economic one. Once back inside the European bloc, an independent Scotland could face a trade border with England, its biggest economic partner.
“I think the S.N.P. spent so much time recently talking about their demand for a referendum and very little time explaining what independence would involve and how it would resolve some of the very challenging questions on pensions, on currency, on relations with the rest of the U.K. and on implication of an independent Scotland in the E.U.,” Professor Mitchell said.
Professor Keating too argues that winning over more wavering Scots and reaching out to other pro-independence groups should be the priority, if Ms. Sturgeon’s successor hopes to shift the debate.
“If you got support up to 60 percent, people would have to take it seriously — and that is all about explaining what it is about, doing more of the homework, engaging more with the social movements and trying to build a broader coalition,” he said.
“I think this is an opportunity for a new leader to rethink the strategy and to re-engage, but fundamentally there is not much room for maneuver.”