Long-Bailey tweeted an interview with actor Maxine Peake, which made an unsubstantiated claim about US police officers being trained by the Israeli secret services.
He replaces Jeremy Corbyn, who led Labour to its worst general election result in 84 years in December last year.
The Labour leadership candidate reportedly said she would give Jeremy Corbyn a role in her Cabinet if she becomes prime minister.
Sir Keir Starmer is the only male in the contest.
Rebecca Long-Bailey and Richard Burgon are close allies of current leader Jeremy Corbyn and shadow chancellor John McDonnell.
'If you have a deterrent you have to be prepared to use it.'
'Two northern women will not be told that we are continuity anything from any man.'
Politicians and high-profile supporters of the outgoing party boss have been publicly promoting Rebecca Long-Bailey.
Corbyn ally Rebecca Long-Bailey is currently the bookies' favourite.
Rebecca Long-Bailey was responding to fresh anti-Semitism claims against Jeremy Corbyn.