Scottish Labor leader Jim Murphy and Scottish National Party leader Nicola Sturgeon during a televised debate on this month in Edinburgh. (Jeff J. Mitchell/Getty Images)

If ever there was a sure thing in British politics, it was this: During general elections, the working-class voters of this gritty yet grand city straddling the River Clyde would turn out with near-religious devotion to back Labor.

That unassailable bedrock of support explains why the party had special reason to celebrate when Scotland turned down a chance at independence last September. Not only had the United Kingdom been saved, so had Labor’s chances to govern the country.

But seven months later, the party’s decades-long dominance of Scottish politics has collapsed. The nationalists, crushed by their defeat in the referendum, are on the verge of a historic victory. And Scotland has become perhaps the most critical battleground in a campaign that, with only weeks to go, is shaping up as the most unpredictable British election in a generation.

Had it not been for the stunning reversal of Labor’s fortunes in Scotland, the party would probably be headed for a decisive win in its bid to unseat the Conservative Party next month after five years of Tory rule. But now the two parties are running neck and neck.

Labor Party veteran John Robertson and first-time Scottish National Party candidate Carol Monaghan hit the streets for Scotland's upcoming elections. They are competing for northwest Glasgow's Parliament seat. (Griff Witte/The Washington Post)

The Scottish National Party is poised to play Labor’s spoiler. And with neither Labor nor the Conservatives likely to secure a majority on May 7, the SNP could also become the kingmaker on May 8, giving a party that is dedicated to the dismantling of the U.K. the chance to effectively pick the next British prime minister.

In exchange for its support, the SNP is asking a king’s ransom: further powers and money for Scotland, a rollback of austerity policies across the U.K. and an end to a British nuclear weapons program that has been a cornerstone of Western security since the 1950s.

The SNP’s expected emergence as a major player in London represents the most vivid illustration to date of the fragmentation of British politics, with nationalist and fringe ideological movements increasingly occupying ground once reserved for the two parties — one center-right, the other center-left — that have together dominated British political life for the past century.

The phenomenon has echoes across Europe, with radical leftists winning votes in Greece and surging in Spain while the extreme right vies for the presidency in France.

But continental Europe has long years of experience with coalition politics and the panorama of ideologies that result. Labor and the Conservatives, by contrast, have never confronted challenges from the margins like the ones they face today.

The Conservatives are being battered on their right flank by the U.K. Independence Party, which has won a loyal following by championing the rights of the English and calling for an end to mass immigration. The party is expected to wrest a handful of English seats from the Tories.

The SNP’s gains are likely to be far more consequential — the party holds a lead of 20 points or more over Labor in the latest Scottish opinion polls.

“There isn’t a safe Labor seat in Scotland,” said John Curtice, a politics professor at Glasgow’s University of Strathclyde.

That is quite a fall for a party that won 41 out of 59 Scottish seats in the last election, in 2010. But it reflects the tremendous political upheaval in Scotland wrought by last September’s referendum, in which voters rejected independence by a 10-point margin but found along the way that they crave a change from the status quo embodied by Labor.

“What the referendum did was expose the extent to which voters had fallen out of love with a party they supported for years,” said David Torrance, a political analyst who has written biographies of the leaders of the Scottish independence movement. “What the SNP did was keep hold of those voters and not let them drift back to Labor.”

The dramatically altered political landscape has meant a rude awakening for many Labor incumbents who are products of machine, one-party politics and had grown accustomed to winning their seats without bothering to campaign.

In northwest Glasgow, where the city’s fabled shipyards are still the largest single employer, Labor veteran John Robertson hardly hit the streets and still won reelection to Parliament in 2010 by nearly 40 points. His SNP challenger placed third.

This time, Robertson is trailing Carol Monaghan, a first-time candidate and mother of three who quit her job as a high school physics teacher to pursue a career in politics after being inspired by the foment of last year’s independence push.

“Post-referendum, there were a few dark days of people feeling sorry for themselves,” said Monaghan, 42, as she sat in a makeshift SNP office buzzing with volunteers one recent evening. “But then I realized that we have to keep going. And although 55 percent voted no in the referendum, what a lot of them were really saying was, ‘Not yet.’ ”

That view is borne out on the campaign trail, where even die-hard Labor voters say they are ready to give the progressively minded SNP a shot in London and to ditch a party they say has become out of touch with the needs of working-class Scots.

“All my life I voted for Labor because my family did,” said Pauline Wright, a 65-year-old retiree.

But after the death of her husband, a former shipyard worker, Labor didn’t lift a finger to help as she struggled through the bureaucratic morass of collecting his pension, she said. When Monaghan knocked on the door of Wright’s modest stucco home, Wright invited the SNP candidate in for tea.

“I’m good with money, but I’m really struggling. And I blame Labor for that. Working people are getting hammered,” said Wright, who declared that Monaghan had won her vote.

The reception one recent spring evening for Robertson, the Labor incumbent, was decidedly chillier. Dressed in a dark suit, the 62-year-old grandfather of two marched through streets lined by neatly trimmed yards and brown picket fences, only to find voters had already made up their minds. And they weren’t voting for him.

“I’m not going to be persuaded,” said one man, a retired janitor, as he shut the door upon seeing Robertson’s face on the other side of it.

“SNP! SNP!” a group of men chanted out the car window as they drove by.

Reading a text off his phone, a young Robertson volunteer reported that other Labor activists had been “threatened with a chain saw” that evening.

“So this,” he said with a grim smile, “has been fine.”

Labor’s trump card with wavering voters has been that by backing the SNP, they will hand another term to the dreaded Tories. The argument has worked in the past, keeping left-leaning voters in the Labor column during general elections even after they had defected to the SNP in local and Scottish votes.

But that appears to be holding little sway this time, given that neither major party is likely to secure an outright victory.

SNP leader Nicola Sturgeon has sought to assuage voters by vowing to “lock out the Tories” and help make Labor leader Ed Miliband the prime minister in the event of a hung Parliament. But she has also named a high price for her support and has not ruled out another referendum on Scottish independence in the near future.

Analysts say that no matter the outcome, the SNP is likely to emerge from the election in a strong position to push its agenda. That is a remarkable turn for a party that lost the biggest vote of its 81-year history just last September and a potentially worrying one for those who want to keep Britain whole.

“They had prepared for defeat,” said Paul Sinclair, a veteran Labor Party strategist who was active in last year’s unionist campaign. “But we didn’t prepare for victory.”

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