In Labour's Scottish bastion, nationalists prepare an upset

* Labour facing big losses in Scotland

* Scottish nationalists poised to end political era

* Election could bring major changes for Britain

* Glasgow is symbol of Labour's reversal of fortune

By Andy Bruce, Kate Holton and Guy Faulconbridge

GLASGOW/LONDON April 17 (Reuters) - In Scotland's biggest city, nationalists have triggered a once-in-a-century shift in political loyalties that could dash Labour leader Ed Miliband's dreams of winning the May 7 election and thrust secessionist 'kingmakers' to the heart of British power.

The shifting currents in Glasgow, the citadel of Scottish socialism for more than a century, show the seriousness of the nationalists' bid to end Labour's dominance of Scotland, a significant change in recent British political history.

The nationalist challenge could trigger events after the election that threaten the future of the United Kingdom and possibly its membership of the European Union.

Scots voted to preserve the United Kingdom in a Sept. 18 referendum but the once marginal Scottish National Party (SNP) has spent two decades persuading Scots that it is a worthy alternative to Labour, which many voters say has abandoned its Scottish heritage.

"We're recovering from a period where the Scottish Labour party wasn't strong enough, and it wasn't good enough. And that takes time," said Jim Murphy, leader of the Scottish Labour Party.

"We've got less than three weeks to turn it round but I'm confident that we can," said Murphy, a 47-year-old teetotal vegetarian who opinion polls show has so far failed to stem a flood of support from Labour to the SNP.

The destiny of Glasgow's once safe Labour seats will show whether Labour has lost Scotland, a defeat that would scupper Miliband's bid to win an overall majority in the 650-seat London parliament and potentially give the SNP a kingmaker position from which to bargain for more powers for Scotland.

The loss of Glasgow, a party stronghold for so many decades, would symbolise the extent of Labour's decline in Scotland and the emerging supremacy of the nationalists.

Labour won all seven of Glasgow's seats in the British parliament in the 2010 election with apparently unassailable majorities of up to 16,000 votes.

Now (NYSE: DNOW - news) all but one are threatened by the SNP, according to John Curtice, professor of politics at the University of Strathclyde and Scotland's most respected opinion poll analyst.

"Glasgow North East is the safest Labour seat in the country and nobody has yet come up with an opinion poll that suggests it could be lost," he told Reuters. "But everything else in the city is absolutely up for grabs."

GLASGOW 'UP FOR GRABS'

From the banks of the River Clyde, Glasgow's merchants earned fortunes in the 18th Century from tobacco from America and after the American Revolution disrupted trade, they imported sugar from the West Indies.

Glasgow became one of Europe's biggest shipbuilding centres by the 19th Century but by the early 20th Century, the docks of "Red Clydeside" were breeding a radical socialism that spooked Britain's leaders during World War One.

Labour became Scotland's biggest party in the British parliament in 1922 and the last time it lost Scotland in a national election was in 1955. By 2014, 45 percent of Scots would vote for independence.

In a small campaign office in a block of council flats, the SNP candidate for Glasgow Central, Alison Thewliss, is trying to sow the seeds of Labour's defeat.

Asked why so many Scots were turning away from Labour, she said: "They saw Labour joining up with the Tories during the referendum, and it's hurt people actually."

Labour's joint effort with the Conservative Party to urge Scots to reject independence alienated some voters, even those who support the 308-year-old union, she said.

'SENSE OF ENTITLEMENT'

Labour's potential loss of Scotland illustrates the divergence of England and Scotland and what many Scots see as an arrogant London political establishment that has failed to heed or address their needs.

While Conservative Prime Minister Margaret Thatcher moved England rightwards from 1979 to 1990, Scots stood opposed, especially to Thatcher's use of Scotland to test an unpopular poll tax.

Her successors, from John Major to Labour prime ministers Tony Blair and Gordon Brown, broadly accepted Thatcher's legacy while millions of Scots held to their socialist traditions.

Those traditions go back to the dawn of the British Labour movement when Keir Hardie, the Scottish trade unionist born in Glasgow in 1856, helped found the Labour Party.

As Blair moved Labour rightwards to win voters in England and established the Scottish parliament, the party assumed its Scottish flank would be secure.

That opened an opportunity for the SNP, which pitches itself as the social conscience of Scotland opposed to what leader Nicola Sturgeon calls the crumbling institutions of the London elite.

Just 12 years after the establishment of the Scottish Parliament, the SNP would topple Labour, winning a majority in the 2011 Scottish election.

"Labour has, from the 1970s, been regarded as the natural party of government in Scotland," said Stewart MacLennan, 64, a former Labour candidate who now backs the SNP.

But now Murphy is greeted on the campaign trail by SNP taunts that Labour, whose party colour is red, are the "Red Tories", in reference to the Conservatives' nickname.

'REAPING THE WHIRLWIND'

While the SNP has added members since the referendum, opinion polls show its share of the vote has risen just a few percentage points.

What has changed is that Scots now appear to have broken a tradition of voting SNP only in Scottish elections and have now also fixed their sights on the London parliament.

Such a change in voting habits could shake the foundations of Britain's political establishment.

A YouGov (LSE: YOU.L - news) poll on April 8-9 found the SNP had 49 percent support in Scotland, with Labour on 25 percent. It was the biggest lead so far for the SNP and the lowest level for Labour since 2007 in a YouGov poll.

Under a uniform swing, the poll showed the SNP would win 53 seats out of all 59 in Scotland, up from 6 in the 2010 general election, while Labour would win four Scottish seats, down from 41 in 2010, making it Labour's worst result in Scotland since 1918, YouGov said.

The Liberal Democrats would win one seat, down from 11 in 2010, while the Conservatives would hold its one seat.

Losing so many seats would scupper Miliband's chances of leading a majority government and potentially allow Prime Minister David Cameron's Conservatives to win a second term.

Sturgeon has given mixed signals on seeking a second independence referendum but has warned that a vote on Britain's membership of European Union, which Cameron has promised, could trigger another referendum.

"The referendum last year was the culmination of a long process of political disillusionment," said Professor Gregor Gall, editor of the Scottish Labour History Society. "The whirlwind from that is still being reaped for Labour." (Writing by Guy Faulconbridge; Editing by Giles Elgood)