The semi-circular "swingometer" graphic has become an integral feature of television coverage of UK general elections, measuring the sometimes infinitesimal shifts in party allegiance on which governments can rise and fall. Late Friday morning, for the first time ever, the swingometer failed, the digital needle freezing way beyond the furthest point it had ever reached after an almost 40 percent shift from Labour to the Scottish National Party (SNP) in Glasgow North East. The SNP had effectively broken British politics.

Across Scotland, the shift to the SNP was off the charts - often quite literally. The Scottish nationalists went into Thursday's general election holding six of Scotland's 59 seats. Now they have 56.

"The tectonic plates of Scottish politics shifted yesterday - it is a historic result," Scottish National Party leader Nicola Sturgeon said Friday.

The face of the SNP's victory: Nicola Sturgeon

By far the biggest losers in the remarkable SNP surge was Scottish Labour, which went from 41 seats in 2010 to just a solitary member of parliament.

Unprecedented shift

Longtime Labour strongholds across post-industrial Scotland - places that Labour had held, in some cases, for over 80 years - saw unprecedented shifts to the Scottish nationalists. Absolute majorities were overturned; MPs who had never faced a genuine electoral threat during decades in office found themselves defeated, often by young, relatively inexperienced Scottish nationalists.

In Paisley, just outside Glasgow, Labour's shadow foreign secretary Douglas Alexander lost his seat to 20-year-old Mhairi Black, a final year university student. Until Thursday, Glasgow, Scotland's largest city, had never had an SNP member of parliament. Now it has seven.

The SNP has seen a huge upturn in support since last year's defeated referendum on Scottish independence. Party membership has jumped from 25,000 to well over 100,000.

In her acceptance speech, new SNP MP Carol Monaghan for Glasgow North West told supporters: "The referendum rekindled Scotland's self-confidence, and this general election has given us our voice."

Shifting allegiance

Glasgow SNP activist Graham Campbell says that during the campaign the party barely had to convince former Labour voters to switch allegiance to the Scottish nationalists.

Labour was punished for failing to deliver on issues important to Scotland

"Labour voters who were Labour voters last year and had voted 'yes' had switched. They had made their choice months ago. We didn't even have to convince them," he said.

The rise in SNP support is, in large part, a product of last September's referendum but the party also managed to appeal to "no" voters who believed the Scottish nationalists were best placed to "stand up" for Scottish interests at Westminster.

"This is still about independence, but it's also about who do you trust to stand up for your interest," says Campbell. "People trust the SNP as a government to deliver progressive, social democratic reforms."

The SNP's election pitch was aimed squarely at Labour voters. Nationalists said that they would form an "anti-Tory alliance" to lock incumbent Prime Minister David Cameron out of office.

Still on the agenda

But the Conservatives - colloquially known as the Tories - won an unexpected majority in Thursday's general election, which means Scottish nationalists are highly unlikely to play any role in government - although that does not necessarily mean that Scotland will be off the UK political agenda.

"Even if they play no role in a coalition government, SNP MPs will have a transformative effect on other aspects of political life at Westminster, not least through their overwhelming representation on the Scottish Affairs select committee," says Ailsa Henderson, professor of Political Science at the University of Edinburgh and fellow of the Center on Constitutional Change.

"They have won this landslide because voters believe they are best able to ensure a transfer of powers to the Scottish Parliament and best able to represent Scotland's interests at Westminster."

The SNP's success is not just a product of the referendum. Labour was heavily criticized in its once working-class heartlands for joining the politically toxic Tories in last year's pro-union campaign. But the roots of Labour's malaise lie deeper.

Festering frustration

Frustration was festering for decades in what was "Labour Scotland." Stacks of general election votes betrayed dwindling party membership, a deep-seated sense of lethargy and disillusionment at the inability of Labour governments to solve the problems that plague much of post-industrial Scotland.

They could win next time

The SNP success will inevitably reopen questions about Scottish independence. On Friday, David Cameron said he would fulfill pledges for greater devolution for Scotland. But any such deal for more powers for Edinburgh would be contingent on restricting the rights of non-English MPs to vote in the House of Commons, a move presently opposed by the nationalists.

The Conservatives, politically toxic in Scotland, hold just a single seat north of the border. Another Tory administration could push Scots closer toward leaving the three centuries old union with England.

After winning almost half the vote in his Gordon constituency, former SNP leader Alex Salmond said "the Scottish lion has roared this morning across Scotland."

Given the scale of their victory, the Scottish nationalist wild cat is unlikely to go back into its cage anytime soon.