Jim Murphy is facing an uphill struggle to win potentially crucial leftwing support for his campaign to become Labour’s leader in Scotland after the country’s two biggest unions refused to back his candidacy.

The former secretary of state for Scotland, who is on the right of the party, apologised to the people of Scotland for Labour’s repeated failure to listen to their concerns, and for its lack of vision for Scotland, as he launched his campaign in Edinburgh yesterday.

“I want to apologise, because twice Scots have said they didn’t think we were good enough to govern in Scotland – in 2007 and 2011. We didn’t listen to them. That has to change,” said the MP for East Renfrewshire, adding that the party’s problem was not its lack of ideals but a failure to convey any real sense of its relevance to people’s lives. “Let’s be honest, it’s our vision for Scotland. Or more truthfully our lack of vision. We have been rejected and now we need to change.”

Murphy cited the fight to reduce poverty and increase prosperity as twin aims if he became leader. MSPs Sarah Boyack and Neil Findlay are also standing in the contest to replace Johann Lamont, who quit saying that the Westminster party treated Scottish Labour like a “branch office”.

While he is the firm favourite of the bookies, Murphy suffered a setback within minutes of delivering his speech yesterday when Scotland’s biggest union, Unison, came out in favour of the leftwing Findlay, and the second biggest, Unite, said Murphy had to do far more to win its endorsement.

Unison’s chair in Scotland, Gordon McKay, said Findlay offered a “radical new policy approach” that would be welcomed by its members.

“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,” he said.

Union members have a third of the vote in the “electoral college” to decide who becomes the new leader and deputy leader. A third also goes to Scottish MPs, MSPs and MEPs together, and a third to party members in Scotland.

Pat Rafferty, the Scottish secretary of Unite, issued a blunt statement making clear Murphy had a long political road to travel before he would win the endorsement of the union. “On the basis of this speech, it is extremely difficult for them to find much to find hope that Jim Murphy is offering the genuine, positive change in Scottish Labour they seek,” Rafferty said. “We urge him to use the coming days and weeks to give Labour voters much more substance to go on.”

Last week, by contrast, Unite welcomed Findlay into the contest. While the voting system is “one member one vote”, and each member is free to choose whichever candidate they want, the recommendations of the unions’ leaderships will be influential, as they were in the election of Ed Miliband as Labour leader in 2010.

Murphy supporters insisted on Saturday night that they were confident that their man would win the support of a majority of Westminster, Holyrood and EU parliamentarians to win the race. He was seen a crucial figure in delivering a no vote in the Scottish referendum, having toured more than 100 towns, cities and villages to make the case for the union.

Meanwhile, the North Ayrshire and Arran MP, Katy Clark, has became the first person to enter the contest for the deputy leadership, which became vacant when Anas Sarwar followed Lamont in stepping down.

On Thursday, opinion polls by Ipsos/Mori and YouGov suggested Labour faced the prospect of losing most of its 41 Scottish seats to the SNP in next May’s general election – enough, potentially, to deprive it of a majority in Westminster.

The choice of the next Labour leader in Scotland is therefore seen as critical to the party’s chances of forming the next government, as well as its chances of reviving itself north of the border.

Voting in the electoral college starts on 17 November and the new leader will be declared on 13 December.