Sheffield Heeley MP Louise Haigh opened up a key campaign event for Labour challenger Owen Smith by saying he has the "energy to fight the Tories".
Ms Haigh was a supporter of Jeremy Corbyn among the new intake of Labour MPs in 2015 and nominated him to become leader, although she later voted for Andy Burnham.
However this morning at the advanced manufacturing park at Orgreave, South Yorkshire, Labour's youngest MP said people in Yorkshire need a Labour Party that is an obvious "Government in waiting".
Before welcoming Welsh politician Owen Smith to the stage she said: "He has the ideas, the energy and the fight in him to take it to the Tories to fight austerity and most importantly to deliver a Labour Government with an industrial strategy for highly paid, high skilled jobs, investment in the North and across the UK and a low carbon future for us all."
Mr Smith is attempting to oust Mr Corbyn and become the next party leader and describes himself as a radical politician who can become the next Prime Minister.
She said: "Everyday we are out of Government isn't just another day that we could be doing things better than the Tories, it's another day they have to dismantle everything our party has fought for over the decades. It's another day to tear apart the welfare state, to dismantle the NHS.
"Our area desperately needs Labour to be in power. They need to look at us as a Government in waiting."
During the 2015 leadership Ms Haigh wrote on her website that it was important for Mr Corbyn to be on the ballot paper to offer voters the widest possible choice and that there was a strong local desire for him to be nominated.
She said: "I decided the most democratic and representative thing to do would be to nominate Jeremy to ensure members had the opportunity to vote for him that would otherwise have been denied."
However over the past year, many MPs have lost faith in Mr Corbyn's abilities to lead the party and Ms Haigh voted against him in the vote of confidence held in June.
She said Owen Smith is now the best person to challenge the Tories' agenda for the North of England.
She said: "We know the Tories always have and always will let the North down.
"Just weeks after taking power in 2010 they dismantled the Regional Development Agencies effectively taking £2bn a year out of the North. At the same time they cancelled the £100m loan to Sheffield Forgemasters. To even mention the Northern Powerhouse to people around here is little short of insulting.
"From the thousands of job losses in the private and public sectors in the last year alone, the ideological neglect of our steel industry and perhaps most symbolic of their London-centric approach - the closure of the BIS policy making centre in Sheffield and the removal of senior civil servants from Sheffield back down to Whitehall."