Owen Smith has confirmed that he will stand for the Labour leadership.
The MP for Pontypridd, who resigned as shadow work and pensions secretary last month, said Jeremy Corbyn was a "good man" but not a leader who could win an election for Labour.
He declared that he was the "radical and credible" leader who could take Labour back into power, telling ITV Wales that his party was currently "at grave risk of splitting".
He will join Angela Eagle in challenging Corbyn, who received a boost on Tuesday when Labour's National Executive Committee (NEC) ruled that he could automatically be included on the ballot paper.
Smith said he agreed with the NEC's decision.
But his leadership bid has not been universally well-received by Labour MPs.
"I hope someone can talk him out of it," one MP told ITV News' Chris Ship, amid fears Smith will split the vote between himself and Eagle - leading to a Corbyn win.
Another claimed Smith's ego was the "size of a planet".
Smith denied that he had ever been part of "any plot or coup" but said the "decent thing" to do was to fight Corbyn for the leadership in a bid to unite the party.
"At the end of that I will stand behind whoever the leader is - but I hope and I expect it will be me."
"I'm not prepared to stand by and let the Labour Party, the party I love and that has been the greatest force for good in this country, split. It cannot happen."
He vowed: "I will never split the Labour Party. I will be Labour 'til the day I die and I will stick with my Labour Party throughout."
The Labour Party has been engulfed in turmoil since Britain voted to leave the European Union.
Corbyn - who was accused of being half-hearted in his attempts to persuade Labour voters to vote to remain in the EU - has suffered a wave of shadow cabinet ministers and a vote of no confidence.
But the 67-year-old has refused to resign, saying it would be a betrayal to the party's members who helped him to a landslide victory in last summer's leadership election.
Union-led peace talks collapsed last weekend.