DUNDEE Scotland (Reuters) - Gordon Brown, once ranked Britain's most unpopular prime minister in half a century, may yet emerge as the man who convinces Scots to reject independence.
For many English, Scotsman Brown is an unlikely hero.
Often brooding and awkward in front of the camera, the former prime minister led his Labour party to its worst electoral defeat in a generation in 2010.
But in the industrial towns of Scotland where the fate of the United Kingdom will be set by 1 million as yet undecided voters in a Sept. 18 referendum, few rival Brown's influence.
In speeches in towns and cities across Scotland Brown makes a passionate case to stay within the United Kingdom - a G8 power with a permanent seat on the United Nations Security Council.
Brown and an increasing number of others are fearful Britain may be sleepwalking towards a break up.
"Countries can be lost by mistake," Brown, who still speaks with the accent of his native land, told reporters over lunch in London's Westminster parliament.
"Don't allow it to become British politicians versus Scotland, which is how too easily this has been caricatured, because that simply plays into the hands of the nationalists. That's a losing ticket."
Less than 100 days before Scotland's independence vote, Brown's is a sobering intervention for many English.
Polls show Scots are still unlikely to vote for independence but they also show the Better Together campaign's lead has narrowed. The head of that campaign, Alistair Darling, has said the vote will be close.
Supported by Britain's three main political parties, the Better Together message has veered from warnings over the perils of secession to emotional appeals for unity.
Prime Minister David Cameron, whose Conservative party has just one of Scotland's 59 seats in the London parliament, has conceded that his privileged English background and centre-right politics mean he isn't the best person to win over Scots.
That has left the Better Together campaign largely in the hands of opposition Labour, winner of 41 Scottish seats in 2010 and the only party with the local organisation and support capable of checking the secessionist Scottish National Party.
Labour strategists said many of the undecided voters are Labour party supporters who dislike being lectured by English Conservatives.
Enter Gordon Brown, one of very few British politicians Scottish nationalist leader Alex Salmond is said to fear.
"As a patriotic Scot I cannot opt out of a debate and decision that affects children whom I love and people whom I respect and represent, all the more so since we are being asked to make an irreversible decision that will have consequences for generations," Brown said in his treatise 'My Scotland, Our Britain: A Future Worth Sharing'.
The son of a Presbyterian minister, Brown studied at the University of Edinburgh and gained a PhD on the Labour Party's role in driving political change in Scotland.
He has represented a Scottish constituency in the London parliament since 1983 when he shared an office with another new Labour lawmaker, Tony Blair.
It was the start of a relationship that would dominate British politics for more than a decade as Brown coveted and eventually got Blair's job as prime minister in 2007 after a decade as finance minister.
In large parts of the United Kingdom, Brown is a divisive figure. His supporters say he steered Britain through the global financial crisis, his critics say he mismanaged the economy and ran up record state borrowing.
Brown has taken to the road in Scotland in his most active political sortie since his election defeat in 2010.
In speeches he says Scotland's identity and rights are best secured by being part of the union.
He has criticised as "patronising" some tactics used by the Better Together campaign, notably ads featuring Lego figures holidaying abroad - a clumsy attempt to persuade Scots they will be financially better off within the United Kingdom.
Brown's advisers say he always intended to play a role in the final stretch of the campaign. But his criticism of the Better Together campaign points to concerns that pushed the 63-year-old to take an even bigger role.
Better Together's Darling has dismissed any suggestion that he may no longer be in charge.
"I run the Better Together campaign," Darling said.
"It was always intended that (Brown) would come in. He was always going to. That is part of the plan. He has a very powerful voice in Scotland. This was always his plan and what he wanted to do. I was very keen that he would do it."
When asked why Brown had taken a bigger role in the campaign, a senior British official said: "He’s one of the big Scottish political beasts. Brown is a very significant political figure in Scotland in a way that he is not always perceived to be in London."
In Glasgow, Scotland's biggest city and a few miles north from his birthplace, Brown recently filled the Old Fruitmarket, pitching the United Kingdom as the best vehicle to promote social justice and preserve welfare spending.
"I'm asking you to vote 'No', because I'm asking you to cast a vote for social justice," Brown told a packed hall of Labour activists, to cheers and applause.
"Social justice is not advanced by retreating into independence," said Brown.
Nicola Sturgeon, deputy leader of the Scottish National Party, said Brown had failed to implement his ideas on social justice while in office.
"Many people will listen to Gordon Brown and think well, why didn’t he do all of these things that he’s talking about now while he was Prime Minister and Chancellor of the Exchequer," Sturgeon said.
"The reality for so many people across Scotland is not one of pooling and sharing resources, it’s about having resources pulled away from them."
Brown's speech is repeated in Dundee and St Andrews, where he mixes soccer jokes, quotes from John F. Kennedy and Nelson Mandela and pride in the achievements of the Scottish Labour movement to argue for staying in the United Kingdom.
"He was very good. I had to stop a few tears coming," said Kenneth John MacDougall, a retired man from Dundee.
By placing the weight of the Scottish Labour movement behind the Better Together campaign, Brown said he was also seeking to forge a modern interpretation of what it means to be British based on pooling resources for social justice and welfare.
"People are asking what Britain is about and they are asking what Britain is for," said Brown. It's a question he is trying to answer in the week's before vote.
(editing by Guy Faulconbridge and Janet McBride)