NOT for the first time in his career, Tony Blair flew into Scotland on Friday as a row was brewing about the constitution. Sir George Mathewson, the former chairman of the Royal Bank of Scotland, had that morning declared his support for independence, and the press wanted to know the prime minister's reaction.

The Labour leader responded by accusing the businessman of "pure self-indulgence", before adding that the banker's opinions were "absurd". Mathewson, who at the time of Blair's comments was fishing in Perthshire, is said to have borne the remarks bravely.

Mathewson is not the first high-profile business figure to back the SNP. Last year, Ben Thomson, the chief executive of investment bank Noble Group, said he was "not at all afraid" of independence. Crawford Beveridge, the executive vice-president of Sun Microsystems, claimed in the Sunday Herald earlier this year that a separate Scottish state could "focus the minds" of politicians to implement pro-business policies. And Sandy Orr, the City Inn hotel chain tycoon, said independence was a "quite exciting" prospect.

Cash has accompanied the declarations: Sir Tom Farmer, the founder of Kwik-Fit, poured £100,000 into Nationalist coffers; Ian Watson, the Glasgow-born chairman of mining development group Galahad Gold, offered a £50,000 donation, while several others in the business community have written five-figure cheques to the SNP election fund.

However, those generous sums were dwarfed yesterday when Stagecoach boss Brian Souter gave the party £500,000 - their biggest donation ever. Souter is known to be an SNP supporter, but this gift will be a significant boost for the Nationalists' election campaign.

The key question is how such a turnaround was achieved. Why are prominent members of the business community, who are normally instinctively conservative on constitutional politics, backing Salmond and giving him a hearing on independence? What has changed?

Part of the answer lies in the charm offensive launched by Salmond and business-friendly SNP MSP Jim Mather last year. For more than 12 months, they and party economic adviser Jennifer Erickson have made around two presentations a week to business leaders on the Nats' plans for the economy, which include cuts to business rates and corporation tax. More than 1000 people have had the gospel preached to them, and many have become converts.

It was one such gathering, at a hotel on the outskirts of Edinburgh last month, that persuaded Mathewson. The presentation, led by Salmond, impressed the former RBS chairman and led to further discussions with the Nationalist leader last week.

On Friday, Mathewson's views on independence were made known in a letter to a Scottish daily newspaper, coinciding with the prime minister's visit to Scotland. He wrote: "I do not share the fear of independence which is currently being fostered by those who have the most to lose by a change in the status quo and those who see Scotland as a source of safe seats, thus guaranteeing their rule over the United Kingdom "Our votes will choose the new first minister of a parliament which has consistently disappointed since its creation, partially due to the lack of high-quality leadership. The outstanding candidate must be the SNP leader, Alex Salmond."

SNP insiders also say Salmond's intense wooing of the business community stems from his party's disastrous Holyrood election campaign in 1999, which it was well-placed to win 12 months before the poll, but somehow managed to throw away.

Back then, when Salmond still called himself a socialist, the SNP tried to outflank Labour on the left by committing itself to higher personal taxation, a strategy that allowed Gordon Brown to portray the SNP as hostile to business.

Another successful tactic in 1999 was Labour's brutal anti-independence message - under the slogan "Divorce is an expensive business" - which resonated throughout boardrooms north of the Border. The SNP's chances were then sunk when 100 Scottish business leaders signed a full-page newspaper advertisement backing Labour.

"The 1999 campaign was a turning point for Alex and others in the party," said one SNP insider. "After that mauling, we felt that we would never again let ourselves be misrepresented as being anti-business and anti-prosperity. Not having the business community onside makes it very difficult."

Salmond's surprise departure as leader in 2000 furthered the SNP's renewal. John Swinney, his successor, sharpened the party's message on company taxation and encouraged pro-business duo Andrew Wilson and Jim Mather to make the case for a low-tax Scotland to business leaders across the country. Ironically, it was a strategy modelled on the late Labour leader John Smith's "prawn cocktail offensive" in the early 1990s.

"The Wilson and Mather presentations were partly inspired by the modernisation of the Labour party," said an SNP source. "Making a case for a fiscally autonomous Scottish parliament either made business leaders more favourable to independence or simply neutral. It challenged perceptions and drained a lot of the poison out of the debate."

Swinney's legacy to Salmond, who returned as leader in 2005, was to leave behind a party that was taken more seriously among both small businesses and large companies. The Banff and Buchan MP then used this foundation and began sketching out details of a pro-business SNP Executive in his own meetings with industry figures. Creating wealth was flagged up as more of a priority than its redistribution.

The SNP's gradual reinvention as the party of profit has also rubbed off on Labour, whose campaign strategists are currently struggling to land a punch on the Nationalists. In 1999, business leaders queued up to back Labour; now, few are coming forward to endorse a party perceived to be on the way down.

The recent intervention of Rangers chairman David Murray, for instance, was notable for his support of the union, rather than for Scottish Labour. Entrepreneur Willie Haughey may have given unqualified support to Jack McConnell's party, but his status as a leading donor hardly made his endorsement a huge surprise. Even CBI Scotland's attempt to embarrass the SNP by posing 11 "key questions" backfired after the organisation's members warned of the dangers of interfering in Scottish politics.

But the fracturing of Labour's support in the business community is not a recent phenomenon. In 2003, a poll found that of the 100 companies that supported Labour in 1999, only 17 continued to do so. A majority, 55, failed to offer the same public support. Another four years of a Labour-led Scottish Executive has deepened the split within the business community.

Another reason for the apparent tilt towards the SNP is not the message, but the messenger. Several business people contacted by the Sunday Herald contrasted the experience of key Nationalists with the lack of private sector know-how in the Labour ranks.

Salmond, for instance, was an oil economist at RBS before he became a politician. Swinney used to work for Scottish Amicable and Jim Mather, currently the SNP's enterprise spokesman, is a self-made millionaire rich enough to be able to lend his party a six-figure sum.

"The reason I enjoyed the Salmond-Mather show," said one businessman, "is because they know what they are talking about. They have a business background."

Labour, industry figures claim, have no such strength in depth. McConnell's sole foray into the private sector was as a lobbyist for Public Affairs Europe, where the number of clients he pulled in (zero) was outnumbered by the one major sleaze investigation he got caught up in. His deputy, Cathy Jamieson, is a veteran left-winger who opposed Tony Blair's party reforms, while the other members of Team Labour are said to have been responsible for more strikes than business start-ups.

If anything, last week's endorsements confirm the campaign trends of the last 10 months. Since last summer, a combination of positive meetings with businesspeople for the SNP and a steady stream of timely polls and bad news stories emanating from Westminster have made this an election that the Nationalists feel they can win. For the first time in their history, the SNP seem to be winning the propaganda war and edging towards their best-ever result in the polls.

But Labour are still alive, and know enough to realise that old tunes can still impress. McConnell's team claims the SNP's position on the third party right of appeal - some MSPs backed it, others did not - shows the Nationalists still harbour anti-business sentiments. In addition, expect Labour to continue their assault on the SNP's local income tax policy, which they say penalises Middle Scotland.

It might also be wrong to exaggerate the electoral significance of the Mathewson endorsement. Wooing a small but important segment of the business community is one thing, but seducing undecided voters in the central belt is much more of a challenge.

As one Labour strategist put it: "A rich banker coming out for Salmond will not lose us one vote in our heartlands. It gives them credibility, but not a single extra seat."