The new leader of Scottish Labour will be decided within the next three weeks.
Voting is under way in the contest to succeed resigned head Kezia Dugdale in the role.
The ballot has now opened, with party members and eligible supporters having 21 days to make their choice.
The leadership contest is taking place after Ms Dugdale suddenly quit the job in August after two years in charge, making her the third Scottish Labour leader to resign since the independence referendum in 2014.
MSPs Anas Sarwar and Richard Leonard are both vying to succeed her.
Mr Sarwar, a former MP who was elected to Holyrood in 2016 where he is the party’s health spokesman, is seen as the more moderate candidate.
Mr Leonard, a former trade union organiser who also become an MSP last year, is regarded as being on the left wing on the party.
Ballot papers will land in email inboxes today.
Mr Leonard said: “Scottish Labour is at a turning point, we can once again become a party of real change.
“As the ballots drop today, it is clear now more than ever that Scotland needs a united Labour Party committed to real and bold change. And that must be our aim.
“The Labour vote is picking up, but that is not enough. We need to win power, not power for its own sake, but power for a purpose.”
Mr Leonard added: “It is with pride and humility that I accept the nominations from 42 local parties and all bar one of the trade unions to be their chosen candidate to lead the Scottish Labour Party.”
But he warned his supporters against complacency, saying: “The real ballot in this leadership contest starts today. So I take nothing for granted and neither should anyone else.”
Mr Sarwar declared: “I am determined to lead the Scottish Labour Party back to where it belongs – in power.”
He added: “Throughout this contest, only one campaign has put forward a series of bold, radical new policies.
“My plan to introduce a Scottish Child Tax Credit of £10-a-week per child will instantly lift 50,000 kids out of poverty.
“I am the only candidate who supports permanent UK membership of the European single market to protect tens of thousands of jobs in Scotland.
“And I’m the only candidate who has put forward proposals for a more progressive income tax system, which would deliver a tax cut for those earning below the average salary and tax rises for the richest 2 per cent.”