The deputy leadership hopeful attacked those considering a walkout if Ms Long-Bailey is elected, saying it was unacceptable for them to sit as independents if their constituents voted for Labour.
Up to 40 MPs are said to be considering their futures if Ms Long-Bailey, a close ally of Jeremy Corbyn, is named as the new leader in April. One MP previously told The Independent that dozens have discussed walking out.
It comes after eight Labour MPs defected last year to form the Independent Group for Change, while several others resigned the party whip in protest at Mr Corbyn’s leadership.
In an interview at her campaign headquarters, Ms Butler told The Independent: “My message would be, if you resign from the Labour Party, then you would be expected to call a by-election in your seat because if constituents voted for a Labour MP, they deserve to be represented by a Labour MP.”
The Brent Central MP, who describes herself as the “unite candidate”, added: “I think that’s fair, proper, just and it’s democracy.”
Ms Butler put discipline at the heart of her deputy leadership pitch, arguing that Labour lost control of its messaging during its disastrous 2019 election campaign.
She said: “The disaster was trying to give everybody the whole manifesto, all in one go.
“I call it the Toby Carvery manifesto – where the plate is just full of stuff and then we are coming in and pouring gravy on top of it, and you are like, ‘Hang on a minute’.
“I got tired of receiving notifications of a new policy. I hadn’t read the one from yesterday.”
The shadow women and equalites secretary also vowed to end the “old boys club” in Westminster and to bolster Labour complaints procedures on antisemitism and sexual harassment.
“The party system was not fit for purpose in the 21st century. It’s got a lot better, but it needs to be even better,” Ms Butler said, arguing that victims should be afforded greater transparency on how their complaint was handled.
Ms Butler acknowledged that “bridge-building” needed to be done with the Jewish community after the antisemitism row that has dogged the party over the past few years.
Fresh from a meeting on the subject – bearing cakes she had been given – Ms Butler said there were still “outstanding concerns” over allegations of anti-Jewish hatred in Labour ranks.
She said: “There has been a deep sense of worry and concern about some unacceptable behaviour in the Labour Party and how it had been dealt with.
“There are still some outstanding concerns and so there needs to be transparency.”
Ms Butler defended herself from criticism for failing to sign up to 10 pledges on tackling antisemitism by the Board of Deputies – along with fellow deputy contender Richard Burgon.
The other three candidates – Rosena Allin-Khan, Ian Murray and Angela Rayner – have all signed the pledges.
Ms Butler cited data protection concerns over requests to see existing cases, adding: “The Board of Deputies is an outside organisation that is not affiliated to the Labour Party.
“So I just requested a conversation so that I can clarify just a few of these points before I signed the pledges.”
First elected to parliament in 2005, Ms Butler has spoken about ongoing racism she faced, including being mistaken for a cleaner and escorted out of the MPs’ tearoom by a police officer.
“When I first came in in 2005, people kept calling me Diane [Abbott] because there was just two of us,” she said. “You kind of get used to it. You don’t always challenge it.
“People say, ‘You’re always talking about race’. I’m like, ‘Number one, I’m a black woman and number two, I don’t always talk about it actually – sometimes I’m forced to talk about it because of instances.
“Actually if I talked about every single incident that happens I wouldn’t get anything else done.”
The senior Labour MP, who wants to be the first black person in a leadership role, feels a sense of responsibility to step forward on behalf of younger MPs and young people outside of parliament.
A meeting with a group of young people, who said her presence on the frontbench had inspired them, played a part in her decision to enter the deputy leadership race.
Ms Butler said: “I promised myself there and then that I would take the responsibility seriously.
“I want them to know that they deserve to be recognised, they deserve to be seen, and they deserve to be acknowledged for everything they stand for and everything they do. It’s part of the reason why I am standing.”
She urged Labour members to “be bold”, saying she believed the party and the country were ready for a black deputy leader.
Ms Butler has little time for Boris Johnson, who she accused of “nicking bits of our ideas and policies” on nationalisation.
“You can’t out-Johnson Johnson,” she said. “He feels no way about lying to get himself out of the situation. He feels no way about being racist to appeal to a certain group to get votes, so you can’t outdo him on those terms.
“You outdo him by being truthful and honest and having values.”
She added: “He will say what he needs to say to get himself out of the situation. But his lies will eventually catch up to him and they will come crumbling down.”
Ms Butler, who will serve under any leader, is confident about her chances of becoming deputy – despite the clear lead held by Ms Rayner, the shadow education secretary.
She said: “Unfortunately I’ve never been the frontrunner. But I’ve still won. And so not being the frontrunner doesn’t faze me. I still think I’m gonna win.
“The membership are absolutely amazing... it’s not just a grassroots movement, I’ve got an underground movement that is just building and building.”
Members begin voting at the end of this week for the new Labour leader and deputy leader, with results announced on 4 April.