A new YouGov poll said 63 per cent of Labour members would back the shadow Brexit secretary in the final round, ahead of Ms Long-Bailey on 37 per cent, once other candidates have been eliminated.
Prior to Labour’s first leadership hustings, the survey for The Times gave Sir Keir a 2 per cent boost over the shadow business secretary compared to a recent poll which had him on 61 per cent.
It comes as Emily Thornberry said Labour needed a “battle-hardened” candidate to take the fight to Boris Johnson during a campaign speech in which she described being bullied as a teenager.
Ms Thornberry, the shadow foreign secretary, is struggling to make up ground, with only 3 per cent of members saying they would support her in the first round, according to the new poll.
Wigan MP Lisa Nandy had 7 per cent, while prominent backbencher Jess Phillips won 11 per cent.
The five candidates will battle it out in a hustings in Liverpool on Saturday as they seek the critical endorsements of local parties and affiliated unions and groups to make it onto the ballot paper.
Ms Long-Bailey, seen as the favoured candidate by Corbyn allies, has already won the coveted endorsement of grassroots activists network Momentum and the left-wing Baker’s Union.
However, Sir Keir was given a major boost by securing the support of Unison, the UK’s largest trade union, and SERA, Labour’s green group.
He was also endorsed by Royle Family star Ricky Tomlinson for his campaigning to seek justice for workers – including Mr Tomlinson – who were jailed after builder’s strike in Shrewsbury in 1972.
Ms Nandy won the backing of the National Union of Mineworkers, but Ms Phillips and Ms Thornberry are both struggling to pick up endorsements.
In her campaign launch in Guildford on Friday, Ms Thornberry sought to showcase her left-wing credentials by referencing her efforts to help striking miners and print workers.
The shadow cabinet minister also pointed to her time shadowing Mr Johnson as foreign secretary, when she described him as a “lying, reckless charlatan”.
She told the audience: “I’m standing to lead our party because I want to be the woman, and I know I can be the woman, who stands up and leads the fightback against Boris Johnson.
“And we’re going to need someone tough, someone resilient, someone experienced and battle-hardened to lead that fight. Because we all know this is going to be a long, tough road back to power after the painful and crushing defeat we suffered last month.”
Meanwhile, in follow-up comments she added that she believed Mr Johnson had “all sorts of problems with women”.
“I have spoken to a number of women who have said that the way he deals with them – and they are younger women – is that he flirts with them,” she said.
“He makes light of them and he undermines them. He has problems with a women such as myself. He doesn’t know what to do with me. I think that he doesn’t know how to relate to women.”
Ms Long-Bailey launched her campaign with a pledge to end the “gentlemen’s club of politics” by devolving power out of Westminster and introduce a Green New Deal to unite Labour heartlands.
In a speech in Manchester that paid homage to her Salford roots, she said: “I will fight for a green new deal that ushers in a new era of prosperity, security and wellbeing. I will fight for investment in the low carbon industries of today and tomorrow to secure a liveable planet for future generations and bring new jobs and prosperity to all regions and countries of the United Kingdom.
“I will fight for world class public services that secure our fundamental needs and free us up as individuals, as families, as communities, to pursue the lives we aspire to. I will fight for the rewilding and restoration of our landscapes, which will multiply many times over the natural wealth that belongs to us all.
“And I will fight for a democratic economy. For modern democratic public ownership, so we all share in the tremendous wealth that human ingenuity and hard work can guarantee us all”.
Meanwhile, shadow education secretary Angela Rayner emerged as the clear favourite for the deputy job on 57 per cent, according to the YouGov poll.
Her nearest rival, shadow justice secretary Richard Burgon, lagged behind on 15 per cent, while Dawn Butler was on 12 per cent and Ian Murray and Rosena Allin-Khan were tied on 8 per cent.
Labour’s new leader and deputy leader will be announced on April 4.