Mr Sarwar praised Monica Lennon, who also gained enough nominations from colleagues to stand for the post vacated by Mr Leonard last week.
With Labour still consistently in third place in the polls, Mr Sarwar said he was “not naive” to the task he faces should he win the race, when the result is revealed a little more than eight weeks away from May’s election.
Of Ms Lennon, he said: “I’m not going to talk down any other candidate, I get on well with Monica Lennon. I’m pleased to be running in this contest with Monica Lennon. Monica Lennon has got a huge role to play in the future of the Labour Party.
“I’m purely advocating my strengths and what I can do to bring the Labour Party together and to make us an electable force again.”
Though he admitted it would not be a “quick fix”, Mr Sarwar said that with new ideas and news MSPs, Scottish Labour can make gains, even going so far as to say they could be vying for Bute House by 2026.
“This is going to take time, it’s going to take new people, new ideas, it’s going to take a fresh vision for Scotland and I’m determined to put that work in so we can rebuild from the bottom up, not just the top down, more importantly from the bottom up to sort the Labour Party out,” he said.
In recent years, Mr Sarwar said, Labour has not given members or voters “the Labour Party they deserve”, adding: “I’m determined to change that.”
Despite the ongoing contest, both Mr Sarwar and Ms Lennon united to reject calls from Scottish Tory leader Douglas Ross to form a “unionist coalition”.
Mr Sarwar accused the Tory MP of looking trying to grab attention with the claim he knew would be rejected.
“That’s got more to do with Douglas trying to get some attention for himself rather than a serious proposition,” he said.
The Glasgow MSP said the country still bore “the scars” of the 2014 independence referendum and of Brexit, adding: “The idea that those who have advocated their own form of either Brexit nationalism or independence nationalism are somehow now the great beacons of trying to pull our country together just isn’t credible.”
Addressing both Mr Ross and First Minister Nicola Sturgeon directly, Mr Sarwar said: “Covid has changed the world. If you think we can go back to the same arguments we were having before this crisis, then I’m sorry you’re not putting the interests of Scotland first – you are putting your own political interest first – and I’m not willing to do that.”
Ms Lennon took to Twitter, saying: “That’s very flattering Douglas, but it’s a red card from me.
“See you on the campaign trail where we’ll be fighting for radical social and economic change, not a political agenda that looks after the wealthy few.”
Both Ms Lennon and Mr Sarwar garnered at least four nominations from MSPs or Labour’s sole MP north of the border Ian Murray to progress to the ballot.
Mr Leonard’s successor will be announced on February 27.