Sources close to the MSPs Richard Leonard and Anas Sarwar were quoted yesterday trading criticism over their backgrounds, with Mr Leonard’s backers forced to respond to claims that being English could harm his chances.
Meanwhile, Mr Sarwar is facing attacks over his family’s wealth, and both men are likely to be questioned over the fact that they attended private schools.
Martin Whitfield, the East Lothian MP who was born and raised in the north-east of England, said the background of candidates was an “irrelevance” and said the briefings should have no place in the contest.
Fears have been raised that the leadership contest prompted by Ms Dugdale’s surprise resignation could turn into a bitter proxy battle between supporters and opponents of Westminster leader Jeremy Corbyn.
A meeting of Labour parliamentarians on Friday is understood to have heard an appeal for calm from interim Scottish Labour leader Alex Rowley MSP, who also asked both sides not to give briefings to journalists attacking their opponents.
That warning appeared to go unheeded, with an unnamed supporter telling a Sunday newspaper: “People aren’t questioning Anas Sarwar’s suitability to lead as a second generation Pakistani-Scot, so why raise this about Richard?”
Mr Leonard’s background is also reported to have drawn comment from senior Scottish Conservatives.
Mr Whitfield told The Scotsman that questions about the background of candidates amounted to “xenophobia”.
He said: “I would be deeply, deeply disappointed in anyone who tried to use someone’s place of birth as a reason to or not to vote for someone.
“It’s about how the person in front of you answers the questions.
“There should be a bit more realism, and a bit more discussion about what someone is going to offer for the leadership.
“If you can find criticism of that, that’s fine and well, but to judge someone by where they were born – which was after all a decision for their parents, if it was a decision at all – that can’t be acceptable.
“The candidates should stand on what they do, what they say, and where they propose to take the party here in Scotland.”
Mr Whitfield added: “It’s not welcome anywhere, in any part of society. It’s an irrelevant piece of nonsense.”
Mr Leonard and Mr Sarwar are widely expected to contest the leadership race between themselves.
Mr Leonard, a trade union organiser for the past 20 years, is widely seen as the choice of the left, while Mr Sarwar, a former MP, opposed Jeremy Corbyn’s re-election as UK Labour leader.
Setting out their stalls in articles for a Sunday newspaper, both men called for party unity.
Mr Leonard said he was committed to delivering the “radical agenda for social and economic change” set out in Labour’s general election manifesto by “extending public ownership, ending austerity, properly investing again in our public services and to redistributing wealth and power to the many from the few”.
He said it was “important to act according to Labour principles rather than simply manoeuvre and position to win support”, and called on the party to be “audacious” in its approach to regaining power.
“We need industrial reconstruction right across Scotland, from the Highlands to the Borders,” Mr Leonard said.
“And we need an economy where we see a fundamental shift in power from those who own the wealth to those working people who create it.”
Mr Sarwar hailed the former Labour leader’s “passion” and “burning desire”, praising Ms Dugdale for her leadership in the wake of the disastrous 2015 general election when “many Labour members wanted to run and hide”.
“The contest will not be about the direction of the UK Labour Party – we are all united in our desire to elect Jeremy Corbyn as our next prime minister. It will be about electing a leader who can serve our country as the next first minister of Scotland,” Mr Sarwar said.
“The next Scottish Labour leader must unite our movement, continue to build confidence among voters and ensure the party are back where we should be.”
One of Scottish Labour’s biggest donors has backed Mr Sarwar to lead the party, saying the Holyrood health spokesman is a “natural leader” and a “potential first minister”.
Property tycoon Alan Massie, who has donated £400,000 to the party, said: “If Anas stood, Anas has got lots of qualities to bring the party together.”
Ms Dugdale shocked party members in Scotland when she announced last Tuesday night she was stepping down from the job with immediate effect.
She had been leader of Scottish Labour for two years, taking over from Jim Murphy after the disastrous 2015 general election campaign.
The Scottish Labour Executive Committee is set to meet on 9 September to plan the contest. The campaign is expected to take as long as two and a half months, with upwards of 30,000 Labour members and affiliated supporters entitled to a vote.
Faced with the possibility that the leadership election will be contested solely by men, senior Scottish Labour figures are understood to be considering changes to the party rulebook to encourage women to stand in future. Former UK Labour deputy leader Harrier Harman last week warned that the top job in Scotland should not “revert back to men”.
Leading female parliamentarians including Jackie Baillie and Monica Lennon have ruled themselves out of the race.
Possible measures include introducing a leadership job-share, or guaranteeing a gender-balanced leadership team, including the deputy position.
A Scottish Labour source told Scotland on Sunday: “There are people who are not averse to starting a debate on this and this would seem to be a good time to do it. In the past people have talked about the possibility of having a job-share leadership.”