Scotland’s nationalists wasted no time: Just minutes after the country’s leader, Nicola Sturgeon, called on Monday for a new independence referendum, a website went live asking people to show their support on Twitter and donate to the campaign.

By Tuesday morning, 204,345 pounds, or about $249,000 — more than a fifth of the £1 million target — had been raised; pro-independence banners in Scotland’s blue-and-white colors had gone up across the country; and celebrities were offering support, including the actor Alan Cumming, who shared a Twitter post by Ms. Sturgeon, with the comment “It’s showtime!”

It was an early glimpse of the Scottish nationalists’ formidable campaign machine, evidently little diminished since the last referendum, in 2014. Support for independence rose from about 27 percent at the start of that campaign to 45 percent at the final count.

Since then, opinion polls suggest, support for independence has edged up again, leaving Scots split almost evenly. Nationalists seem convinced that they can win this time, thanks to Britain’s vote to leave the European Union, known as Brexit. And unlike Prime Minister Theresa May’s government in London, which is already stretched by the monumental task of negotiating a divorce settlement with 27 other European governments, Ms. Sturgeon’s troops are ready: Party membership quadrupled after the last referendum and increased further after the decision to quit the European Union.

That, one senior British diplomat said of the looming fight with Scotland, is the last thing London needs.

Adding to Westminster’s troubles, on Tuesday the leader of Sinn Fein, the Irish nationalist party, called for a referendum on Northern Ireland’s leaving the United Kingdom and joining Ireland “as soon as possible.” Even Welsh nationalists, sniffing an opportunity, began discussing independence.

Holding a legally binding referendum in Scotland would require the approval of London. Few expect the authorities in Westminster to reject the request, but they could delay the timing, which some say would reduce the likelihood of a vote for independence.

The last referendum was billed as a once-in-a-generation event, but the decision to leave the European Union has changed that. Sixty-two percent of Scots voted to remain in the bloc, making Scotland more pro-European even than London.

The result crystallized a long-held feeling among Scots that a right-wing Conservative government in London did not represent them and their more-progressive sensibilities. And with the opposition Labour Party in crisis, the Conservative Party looks set to remain in power in London for years — possibly “until 2030 or beyond,” as Ms. Sturgeon put it on Monday.

“Scotland hasn’t voted Conservative in decades and yet has been ruled by a Conservative government for most of that time,” said Mhairi Black, a Scottish National Party lawmaker in the British Parliament. “We voted against Brexit, and people are feeling the injustice of it.”

The call for a new referendum has pitted Ms. Sturgeon, the pugnacious leader of Scotland’s semiautonomous legislature, dominated by her Scottish National Party, against the tenacious prime minister, Mrs. May, whose Conservatives have an absolute majority in the British Parliament.

It has also set two radically different brands of nationalism and economic policy against each other.

As the language of national identity gains traction in the West, Scotland is no exception. But the Scottish brand of nationalism looks very different from the far-right varieties that have sprung up elsewhere — not least in neighboring England, where the U.K. Independence Party has been the noisiest supporter of Britain’s withdrawal and part of the reason the once pro-European Conservative Party now takes a hard line on immigration, too.

Rather than an exclusive nationalism rooted in ethnicity, Scottish nationalists speak of an inclusive civic nationalism that can accommodate American-style hyphenated identities.

Humza Yousaf, Scotland’s transportation minister and the son of Kenyan and Pakistani immigrants, summed it up: “It doesn’t matter where you come from, what matters is where we’re going as a nation. You can be Pakistani-Scottish, Polish-Scottish, even English-Scottish.”

For Ms. Sturgeon and Mrs. May, it is a winner-take-all political challenge. Ms. Sturgeon knows that she would need to win a new referendum; another loss would bury all hopes of secession for the foreseeable future. Nationalists point to the French-speaking Canadian province of Quebec, where two independence bids failed, diminishing a once-vibrant secessionist movement.

And Mrs. May certainly does not want to become the prime minister who lost Scotland. Taking office after Britain’s vote to leave the European Union, she sought to strike a conciliatory tone, making Edinburgh her first official visit and promising Ms. Sturgeon a partnership.

But in recent months, Scottish efforts to secure a special status in the withdrawal negotiations — such as access to Europe’s single market — have fallen on deaf ears. “Our efforts at compromise have instead been met with a brick wall of intransigence,” Ms. Sturgeon said on Monday.

A measure of how icy relations between London and Edinburgh have turned is that Ms. Sturgeon’s call for a new referendum seemed to catch Mrs. May by surprise — and might have been the reason she postponed an expected announcement on starting European exit talks on Tuesday.

Mrs. May’s frustration was evident. Accusing her Scottish counterpart of “tunnel vision,” the prime minister dismissed the proposed timetable for a Scottish referendum, in the autumn of 2018 or the spring of 2019, as “the worst possible timing.”

Ms. Sturgeon wants to hold the vote toward the end of the two-year withdrawal negotiations that are to start this month. By that time, the outlines of a deal — one that would almost certainly leave Britain, and Scotland, outside the free trade area of the single market — would have become clearer. But Britain would not yet have left the European Union.

Officials in London would prefer to postpone a vote until after the exit, when the Scots would face the prospect of being outside both Britain and the European Union.

As speculation mounted that Mrs. May might delay a vote until after the next Scottish elections in 2021, Ms. Sturgeon hit back on Twitter, reminding the prime minister that she had yet to win an election. “I was elected as FM on a clear manifesto commitment re #scotref. The PM is not yet elected by anyone,” she wrote, referring to her role as first minister.

Before the last independence vote, Scots were told by London that the surest way to stay in the European Union was a vote to stay in the United Kingdom. Now that it has turned out to be a one-way ticket out of Europe, some Scots who opposed secession before have switched sides.

There is no guarantee, though, that an independent Scotland would be allowed to remain in or rejoin the European Union. Spain appears likely to object, out of concern over setting a precedent that would lift separatist movements in Catalonia and the Basque region. Already, Spanish officials quoted in the Scottish news media have said that Scotland would have to join the “back of the queue” for membership talks.

Privately, some Scottish nationalists say they worry about this, too, which is why the idea of Norway-style access to Europe’s free trade area, rather than full membership, is being discussed in some circles.

However things work out, there is an unmistakable note of destiny building in nationalist circles.

Three decades ago, Scotland’s independence movement was little more than a fringe voice of romantic protest. Only one in four Scots voted for the nationalists. They have been galvanized above all by a sense of being ignored and patronized by the British Parliament, from the long and unpopular government of the former Conservative prime minister, Margaret Thatcher, through to the vote to leave the European Union.

The Thatcher era, which politicized a generation of Scots and recruited activists like Ms. Sturgeon to the nationalist cause, changed Scotland, said Catriona MacDonald, a historian at Glasgow University. The political nationalism of the Scottish National Party is symptomatic of the Scotland that grew out of that time, she said. “Surviving the Thatcher years emboldened Scottishness but it didn’t essentialize it, it made it more accommodating — the opposite of UKIP,” Ms. MacDonald said, referring to the U.K. Independence Party.

In theory, the cards should be stacked against independence.

Scottish North Sea oil and gas revenues have plummeted since the last referendum on independence, growth has slowed and uncertainty about the European Union has deepened. Scotland receives more in payments from London than it sends back in taxes. The question of what currency Scotland would use if it became independent — a prominent question in the last referendum campaign — remains unsolved.

But as the economic case for independence has weakened, for many, the emotional and political case has strengthened.

Ms. Black, who has said that she may not run for re-election to the British Parliament because she found it “depressing,” said she was angered by the idea “that you should vote against independence because the U.K. economy will suffer after Brexit.”

These are fast-moving times and “with independence comes control,” she said. “Would you not rather know that when something happens you are able to respond instead of being at the mercy of a government you didn’t vote for?”(NYT)