The race to lead Scottish Labour has intensified with further union backing for left-wing candidate Neil Findlay and visible support from elected members for centrist Jim Murphy.

Scotland's largest trade union Unison has joined transport union Aslef in backing Mr Findlay, while Unite poured scorn on Mr Murphy's campaign launch in Edinburgh in this morning.

Former Chancellor and Better Together chairman Alistair Darling turned out in support of Mr Murphy, alongside MSPs John Pentland, James Kelly and Hanzala Malik and Glasgow City Council leader Gordon Matheson.

Mr Murphy said he wants to reclaim Labour as "the national party of Scotland", insisting he will hold all of the seats that two polls last week predicted Labour would lose spectacularly, and reverse the growing appetite for another independence referendum.

Separate polls for The Times and STV predicted Labour would lose a quarter of their Scottish MPs at best, and could be on course to lose 90% leaving a rump of just four.

Yesterday, Mr Findlay insisted Labour is in trouble unless it comes forward with policies that people "recognise as Labour".

Unite said today that they are finding it "extremely difficult to find much hope" in what Mr Murphy is offering so far.

Mr Findlay has set himself against Trident nuclear weapons and expressed sympathy for full devolution of income tax, while Mr Murphy has spoken out against unilateral disarmament and remained cagey over his own fiscal devolution proposals.

Mr Murphy pledged to reveal more about his tax plans in the coming weeks, but he dismissed former prime minister Gordon Brown's warning that full devolution of income tax is a "Tory trap" designed to strip Scottish MPs of their voting rights.

Speaking in Edinburgh, Mr Murphy said: "At our best we have always been the national party of Scotland. Many Scots need us back - and I'm here to say we're coming back."

Further data from The Times poll this morning suggests 52% of people in Scotland now back independence, with the STV poll finding that as many as two-thirds of Scots want another referendum within 10 years.

Mr Murphy said: "There is a sense that politicians have to go on and deliver the degree of devolution, we have to honour our commitments in the referendum so that we have a more powerful and influential parliament.

"I think once that's done these numbers will change, of course."

Looking ahead to the general election, he said: "We will win all of the seats we currently have - what we have we will hold.

"I know how we can do that. It's going to take a lot of work, I'm confident we can do it and that will be a great springboard for 2016 when I know with even more work, a great deal of passion and a love of our country we can have the determination to change what is wrong with our country.

"If I didn't believe we can win in 2015 and 2016 I wouldn't be standing for leader."

He added: "I'm not going to do the Alex Salmond thing of going up and down and making multiple journeys between the two parliaments.

"The fact is things have changed in Scotland, Holyrood is changing, more decisions are going to be taken here in Scotland and I want to be part of that."

Mr Findlay, who attended a union hustings in Glasgow today, said: "The vision Labour needs is one of creating jobs, building houses and providing college places.

"We should be listening to the people - like them I want an end to youth unemployment, to address the crisis in social care and provide working people with a fair wage and dignity through a decent job .

"That's the vision I think that the Scottish people want to see "

Unison Scotland Labour link chair Gordon McKay said: "Members have been hugely impressed with Neil Findlay since he became an MSP and in particular as shadow Cabinet secretary for health and wellbeing. Neil understands that politics as usual isn't good enough and we believe he offers a fresh approach with a real experience and understanding of the concerns of working people".

"At today's meeting Neil Findlay outlined a radical new policy approach that will be welcomed by our members."

Pat Rafferty, leader of the Unite union in Scotland, said: "Mr Murphy needs to put away his Irn Bru crate and start setting out what he stands for.

"This is an election about who can best deliver for working and community Scotland.

"We sincerely hope it will not be much longer before Jim Murphy tells us what policies he is promoting.

"Unite's members want to know what he will do to reverse falling wages, attack poverty and defend our services. What matters is, whoever succeeds, what he or she will do in power.

"Unite's representative members will soon decide who to nominate on behalf of our union. On the basis of this speech, it is extremely difficult for them to find much hope that Jim Murphy is offering the genuine, positive change in Scottish Labour they seek.

"We urge him to use the coming days and weeks to give Labour voters much more substance to go on."

Meanwhile, the SNP has called on Mr Murphy to apologise for voting for the Iraq war.

SNP MSP Sandra White said: "The Iraq invasion was New Labour's war - driven by Tony Blair and underpinned by a dodgy dossier and the false prospectus of weapons of mass destruction. Can Jim Murphy at long last apologise for his support for the Iraq war?

"If Mr Murphy cannot bring himself to apologise for his party's biggest failing, then any apology he makes for lesser matters will ring hollow."

North Ayrshire MP Katy Clark has become the first candidate to declare she is standing for the deputy leadership of Scottish Labour.

Ms Clark has already received the backing of rail union TSSA, which is also backing Mr Findlay in the leadership race.

Ms Clark, who has been an MP since 2005, said: "I have submitted a declaration to the Scottish Labour Party that I intend to stand for deputy leader and will be seeking nominations from colleagues over the weekend.

"I wish to pay tribute to both Johann Lamont and Anas Sarwar for the leadership they have provided to Scottish Labour over the last three years. This, however, is not about individuals. We must recognise that Scottish politics has changed and it can't be business as usual.

"The vast majority of people want a secure job, a decent home and access to good quality public services. This is prevented for too many by wealth being held in the hands of a minority.

"It is unacceptable in the 21st century that people have to queue at a food bank to feed their children. These are the issues I will be addressing throughout the campaign. I will make a full statement on Monday."

Manuel Cortes, general secretary of TSSA, said: "Our union is extremely proud to be backing Neil Findlay and Katy Clark for leader and deputy leader of the Scottish Labour Party.

"It is clear that whatever way people voted in the recent referendum, they all wanted radical change. Labour won't be able to win Scottish hearts and minds unless it ditches failed austerity and neoliberalism.

"Neil and Katy represent the workable alternative to the market driven madness that is causing so much harm to ordinary people in Scotland and across the UK."