Scottish Labour must stop blaming the SNP for everything that’s wrong with national politics, according to one candidate for leadership of the party.

Launching his formal bid to succeed Jim Murphy, who officially steps down later this month, Ken Macintosh warned that Scottish Labour had become “a very negative party ... in danger of being defined by our opposition to things rather than what we stand for”.

You cannot say that every single thing that’s wrong in Scottish politics is the fault of the SNP Ken Macintosh

He said he would be prepared to campaign with the SNP on the EU referendum and would not rule out sharing a platform with pro-European Conservatives, despite the damage that a similar strategy did to his party during last year’s Scottish independence referendum campaign.

Asked about UK Labour leadership candidate Andy Burnham’s warning on Tuesday that the party should heed the lessons of the independence referendum and campaign separately from the Tories on Europe, Macintosh said: “I think that’s the wrong lesson. We undoubtedly suffered from the way that we were portrayed in the [independence] referendum, but I don’t think we can stand with the SNP and not be prepared to stand with Tories [who are pro-European].”

Promising to work in a “progressive alliance”, the phrase used regularly by the SNP leader, Nicola Sturgeon, during the general election campaign to indicate her willingness to work with those of similar political outlook across the UK, Macintosh said: “I will go out of my way to make common cause with other parties.”

The MSP for Eastwood, who lost to Johann Lamont in the 2011 leadership contest, said: “The SNP has adopted a lot of our policies; it’s not difficult to agree with them. So why don’t we work with them where they are actually trying to be progressive? That doesn’t mean not opposing them, it means not being defined by opposition.

“You cannot say that every single thing that’s wrong in Scottish politics is the fault of the SNP.”

Defining himself as the candidate of reform, Macintosh said: “The perverse opportunity offered by our terrible defeat is that with only one MP in Scotland, we actually have the opportunity to start again.”

The perverse opportunity offered by our terrible defeat is ... we actually have the opportunity to start again Ken Macintosh

He argued that radical change must begin within the party itself, which needed to be “less hierarchical, less representative of sectoral or vested interests and more open”.

Backing Jim Murphy’s call for one member, one vote in leadership contests, which is already party policy for the UK party, Macintosh went further, calling for an open primary system. He said: “If you want to ask voters to trust you then you’ve got to show that you trust them. What better way of illustrating it than asking them to choose your leader?”

Macintosh will face Scottish Labour’s deputy leader, Kezia Dugdale, who has received the backing of a number of influential MSPs in recent weeks, and earlier on Wednesday from former leader Johann Lamont, who described her as “part of the post-referendum generation” who can “give Labour a strong voice, a modern voice in our new Scotland”.

Giving her first broadcast interview on Tuesday since her resignation last November, Lamont told BBC Radio 4’s Today programme: “We need to elect a leader not on a short-term contract, but a long-term appointment for the long haul.”

Last month, Macintosh complained that his supporters were being “bullied and intimidated by the party machine” in an attempt to prevent a leadership contest, but insisted on Wednesday that this was “no longer a concern”. Referring to “over-zealous behaviour”, he said: “It’s not for people within the parliamentary party to decide whether there is a contest or not.”

Macintosh added that he could understand why some people did not want another contest and were demoralised after the bruising referendum campaign and devastating general election results, but said tht “this is not the time for the path of least resistance”.