Angela Eagle has launched her bid to become deputy leader of Labour with a warning that the party must not collapse into an internal war.
Harriet Harman, the current deputy, has announced she will step down from the position once the party chooses a replacement for Ed Miliband.
Other Labour MPs in the running for the position include Stella Creasy, Tom Watson, Caroline Flint and Ben Bradshaw.
In a YouTube video posted on Monday morning, the former shadow leader of the House of Commons said she wanted to “unite the Labour Party after a heart-breaking election defeat".
Writing for The Huffington Post UK, Eagle warns Labour it must not "indulge in any more of the faction fighting and score settling we've seen this weekend" as "only our political enemies will rejoice if this carries on".
Tensions over the future of the party burst into the open over the weekend after Jim Murphy declared he was resigning as Scottish leader and delivered a stinging parting shot at Unite boss Len McCluskey as a political "kiss of death".
Eagle, who was a Treasury minister in Gordon Brown's government and has also served as shadow chief secretary to the Treasury, will highlight her economic expertise. The party's perceived failure to be seen as economically credible has been widely blamed for its failure to secure seats across England.
Launching her bid, the Wallasey MP said: "Labour must be the answer to a prosperous and successful economy in a twenty first century world where past success is no guarantee of future fortune. This means the whole country being rich with opportunity north and south, east and west.
"We have to challenge current economic orthodoxies and trickle-down economics head on. We must never be against wealth creation but we can be against tax evasion and we can insist on fair rules for those at the top as well as those at the bottom of the income scale.
"Labour must also be at the forefront of building a new economic settlement rooted in rewarding innovation and skills. This new economy needs to be based on developing opportunities for all our people."
Today, Harman pledged that the leadership contest will be opened up to the public for the first time, with televised hustings and questions from non-Labour voters in areas where the party lost seats on May 7.
Supporters of Liz Kendall, one of the favourites for the leadership, hope that new party members and supporters will be able to McCluskey from dominating the contest.
Labour MPs Yvette Cooper, Andy Burnham and Mary Creagh are also running to succeed Miliband. Tristram Hunt is due to announce this week whether he will join the race. Chuka Umunna surprised Westminster on Friday when he pulled out of the contest.
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