Labour leadership: Corbyn and Smith clash on winning power
Jeremy Corbyn has insisted he can reach out beyond his core supporters as his leadership rival said Labour was not on a "trajectory to win power".
At a hustings event, Mr Corbyn said his policies would win over "some people that have been tempted to vote Tory".
He also would not commit the UK to providing military help to a Nato ally, should it be invaded by Russia.
Challenger Owen Smith said he did not believe Labour would win power under Mr Corbyn's "ineffective" leadership.
At the hustings in Birmingham, Mr Smith said: "I feel right now we are not about to win in Kingswood and Milton Keynes and Cardiff North and all of the 116 seats we have got to win from the Tories.
"And the choice for this generation is: Are we happy to sit back where we are or do we think we need to be a Labour government once more? I think we need to be a Labour government once more and I think we need to be a Labour government very, very quickly."
The former work and pensions spokesman is challenging Mr Corbyn for Labour's top job, after the leader lost a no-confidence vote of his MPs.
Mr Smith said both he and Mr Corbyn agreed on many issues but disagreed on whether the party was on a "trajectory to win power" under the current leadership.
But Mr Corbyn said his message appealed "all across the piece, all across the country".
"If you are poor and up against it in Milton Keynes, in Shrewsbury, or anywhere else you are poor and up against it, therefore the message we are putting forward about an economy that doesn't accept the political choice of austerity, instead invests for all, I think is a very powerful one."
He pledged to "win over people... by the policies we put forward", such as decent housing and security at work, including "some people that have been tempted to vote Tory" in the past.
"But we also, I think, win an election by inspiring our own supporters, inspiring those that have supported other parties, but above all reach out to young people in our society, only 47% of whom voted in the last election to come on board with us and try and create that decent society."
Mr Smith questioned Mr Corbyn's leadership style - suggesting he should have pressed former PM David Cameron harder on Iain Duncan Smith's surprise resignation from the cabinet - and PM Theresa May on the implications of the EU referendum.
He added: "I don't just think we have got to win over 'some people' who have been tempted to vote Tory, as Jeremy put it. I think we have got to win over two million people who voted Tory just a few months ago. "We got 9m, they got 11m. I want a Labour government in order to put into practice my principles."
He said the polls suggested, if anything, that more people backed Theresa May than Labour - to some boos. He was also shouted down when he said the reason for the leadership contest was because "170 socialist MPs don't feel that Jeremy is offering leadership" and criticised "uncomradely" behaviour.
But Mr Corbyn stressed that MPs were "there on the sufferance of the people who have put us there" and pledged to continue trying to change the "yah boo" nature of prime minister's questions by putting people's questions directly to Theresa May.
"The response I'm getting outside of Parliament is very positive," he said.
On Nato, which enshrines the principle of "collective defence" - where an attack on one member is considered an attack on all members - the two rivals were asked how they would react if a Nato ally was the victim of aggression from Russia.
Mr Smith said: "We would have to come to the aid of a fellow member of Nato. That would be the job of Britain in the event of a fellow Nato member being invaded, obviously" - while stressing it would be "calamitous and we must never see it happen".
But Mr Corbyn replied: "I would want to avoid us getting involved militarily by building up the diplomatic relationships and also trying to not isolate any country in Europe to bring them up."
Pushed on whether he would become involved if he had to, he said: "I don't wish to go to war. What I want to do is achieve a world where we don't need to go to war, where there is no need for it. That can be done."
Mr Smith's spokesman said after the event that the leadership challenger had been "crystal clear that Britain should live up to our responsibilities" to Nato allies while Mr Corbyn had "refused to provide that same guarantee".
The Labour leadership contest result will be announced on 24 September.