Labour is failing to provide a "powerful, credible opposition" to the Conservative government under Jeremy Corbyn, leadership challenger Owen Smith claimed as the two men clashed in the first head-to-head debate of the campaign.
In an often fiery exchange, Mr Corbyn hit back, blaming Mr Smith and others for undermining party unity when they quit the shadow cabinet in protest at his leadership.
The challenger was booed and heckled by Corbyn supporters at the televised debate in Cardiff as he denied being part of a "coup" to oust the veteran left-winger.
He accused Mr Corbyn of presiding over an increasingly fractious Labour Party and "sloganising" rather than developing effective policies that would lead them back to power.
Addressing Mr Smith, Mr Corbyn said: "What I don't understand is how you can complain about disunity in the party when you and others are the ones who resigned from the shadow cabinet at the very point when we could have taken it to them."
The audience erupted into loud cheers at the Labour leader's remarks but were met with fury from Mr Smith.
The Pontypridd MP said: "Jeremy, I'm not having that because you know, you know Jeremy, that I wasn't part of any coup in the Labour Party."
The leadership contender's remarks were almost drowned out with boos from the audience, which included a number of people wearing "Team Corbyn" t-shirts.
For much of the debate, the two contenders traded promises to fight austerity, boost employment and offer protections to workers.
However, Mr Smith repeatedly stressed he did not think Mr Corbyn would be able to win an election and deliver on his promises to fight inequality.
Challenged on why he resigned as shadow secretary of state for work and pensions, he said: "Because I don't think you're going to be able to deliver it. I don't think we can win, Jeremy, at the moment.
"Without being able to win and put our principles into practice, I don't see that we are going to be able to do anything other than protest.
"We've got to win in order to get this stuff done, otherwise it's just hot air."
He also hit out at criticisms of the 172 Labour MPs who voted no confidence in Mr Corbyn's leadership, saying: "They are not red Tories. These aren't people who want to see the Tories back in power. They are not Blairites. They are just Labour MPs, Jeremy."
The former Labour frontbencher was accused by Mr Corbyn of having "walked away" when the party should be rallying its support and opposing the Conservatives.
Mr Smith also accused Mr Corbyn of failing to do enough to campaign for Britain to stay in the EU or to stamp out anti-Semitism in the Labour Party.
While Mr Corbyn insisted he had campaigned to stay in the EU, he refused to back his challenger's calls for a second referendum on any Brexit deal.
Both contenders won big cheers at the end of the night by promising social and economic reform.
Mr Smith said he would enact "the most radical programme ... since the great Labour government of 1945" and to lead the party back to power.
Mr Corbyn promised to deliver something more than "austerity-lite" and, appealing to his grassroots support, added: "People are engaged in politics in a way they never were before ... (If we) mobilise and enthuse people, we go a lot further and a lot faster."