Shadow foreign secretary Ms Thornberry trumpeted her history of campaigning on abortion rights, an issue where Ms Long-Bailey, a Roman Catholic, has run into controversy.
The shadow business secretary took a swipe at frontrunner Sir Keir Starmer, the shadow Brexit secretary, for playing “procedural games” in Parliament instead of winning over the public.
She vowed to end the “gentleman’s club” in Parliament, another apparent jibe at Sir Keir, the only man in the race.
Today’s launches marked the beginning of the contest’s next phase. The five candidates have secured the endorsements needed from fellow MPs and must now obtain backing from constituency Labour party (CLP) branches and trade unions.
Sir Keir pulled ahead with the support of Unison, the biggest union, and at least six CLPs. Ms Long-Bailey picked up endorsements from Momentum and three CLPs.
Ms Thornberry, who is last in the polls, returned to the Guildford estate where she grew up to make a launch speech that emphasised humble beginnings and “battle-hardened” experience.
“In my 42 years as a member of the Labour Party, there is no fight or campaign our movement has waged where I have not been on the frontline,” she was set to say.
“And since coming to Parliament 15 years ago, I’ve also been on the frontline in the fights against climate change, Universal Credit, and anti-abortion laws in Northern Ireland.”
Her reference to abortion highlighted an uncomfortable area for Ms Long-Bailey, who said in the general election campaign that she “did not agree” with the law that allows terminations on grounds of disability after 24 weeks.
A spokesman for the candidate stressed that she “unequivocally supports a woman’s right to choose” and had voted for extending the right to abortion in Northern Ireland.
The website Red Roar, however, said she had abstained in “a number of parliamentary votes on abortion”, including one relating to Northern Ireland.
Ms Thornberry revealed former Scottish Labour MP Danielle Rowley will run her campaign. Other signings include Cheryl Pidgeon, Unite director in the East Midlands; Momentum and People’s Vote activist Cathleen Clarke; and former North-East MP Helen Goodman.
Ms Long-Bailey will set out her stall in a speech in Manchester tonight, with a vow to “pick a fight with the political establishment”.
Trailing her pitch in a Guardian article, she said: “Instead of winning procedural games in Parliament, we should have used the aftermath of the referendum result to go around the country, holding public meeting after public meeting to stir up a movement for real change.”