Alex Salmond yesterday claimed Labour was losing the election because of its negative tactics and because it lacked anyone of the stature of Donald Dewar.

Speaking at Bannockburn in the battleground seat of Stirling, he said all Labour was succeeding in doing was talking up the SNP.

"The trouble with bogeymen in politics is that if people open the door and see there isn't one there the threat doesn't work as well the next time and doesn't work at all the time after that," said Mr Salmond.

Eight years ago Labour had pushed its "Divorce is an expensive business" campaign early on but then turned to positive promotion of Donald Dewar.

Was he saying the current First Minister suffered by that comparison? "I'm not just saying that Jack McConnell ain't no Donald Dewar. I'm saying he's probably not even Henry McLeish.

"The scare's been tried and now they have nothing positive to say, but you cannot mobilise on negativity alone so Labour have demoralised their own troops and will get their comeuppance."

His attack came as the STUC was finally giving an endorsement to Labour, but not without doubters and claims that a card vote might have produced a very tight result.

Despite one major union abstaining and some voting against, the STUC annual congress voted to back Jack McConnell's party at the polls.

Delegates argued that Labour was the only party with union interests and that the only choice to be made was between Labour and SNP.

Earlier this week the STUC general council voted by one vote to issue a statement endorsing Labour but it had to be approved by delegates at the congress.

The issue sparked the liveliest debate of the three-day conference.

General secretary Grahame Smith said: "Politicians court the approval of Scotland's trade unions because they recognise our standing in Scottish society and our role in promoting progressive change.

"But the same politicians fail to provide the positive agenda for Scotland's workers that would justify trade union support, and the commitment of some to trade union policies and values will last no longer than the election campaign.

"It is the view of the general council that the best opportunity of having the agenda we have set this week implemented is through the election of Labour candidates and a Labour executive."

The statement was opposed by the RMT and some local trades councils while many others, including public sector union PCS, Unison and some teaching unions, abstained.

Gordon Martin of the RMT said: "Jack McConnell gave us 15 minutes of political drivel. He attacked the RMT when signallers were in dispute with Network Rail."

John Quigley of Amicus criticised SNP leader Alex Salmond for taking a seat at Westminster instead of Holyrood, while Margaret Fleming of shop workers' USDAW said: "I have lived through a Tory government and I don't want my family to experience that."

Rail investment was the election battleground of the day, with the SNP proposing to scrap plans for a new station under Edinburgh Airport and spend money on the rest of the network instead.

Mr Salmond said a host of changes improving journey times across the whole country could be achieved for half the £600m earmarked for the Edinburgh Airport plan. He said that plan, and the Edinburgh tram proposals, were deeply flawed.

Bristow Muldoon, Labour's campaign director, said: "It is astonishing that the so-called Scottish National Party wish to abandon projects which would equip Scotland's capital with a 21st century transport system."

In Bannockburn, Mr Salmond won applause for plans to abolish council tax and benefits for the elderly.

The SNP now believes that the Labour seat of Stirling is within its grasp, with candidate Bruce Crawford quoting Robert the Bruce: "To take Stirling is to hold Scotland."