Boris Johnson was forced to defend his plans to cut taxes for high earners in a fractious TV debate with rival Jeremy Hunt.
The Foreign Secretary accused Mr Johnson, the favourite in the battle for the Tory leadership, of sending out a signal that the Conservatives were a “party of the rich”.
The former London mayor insisted his priority was to provide a boost for low earners but defended a package which would also include measures to help people earning more than £50,000.
The ITV debate was the public’s first chance to see the two men fighting to become the next prime minister go head-to-head.
Mr Johnson has been accused by opponents of shying away from debates with Mr Hunt, and he suggested the heated row over tax policy was an example of why he was sceptical about them.
“This is one of the reasons these blue-on-blue debates are so embarrassing,” he said.
The Foreign Secretary shot back: “And you haven’t done any until after people have voted.”
Tory members have already received their postal ballots and Mr Hunt had pleaded with them not to vote until after a chance to see the two candidates in action in the TV debate.
They have until July 22 to cast their vote, with the new Tory leader announced the following day.
Either Mr Johnson or Mr Hunt will take over from Theresa May as prime minister on July 24.
Mr Johnson said he would increase National Insurance thresholds for those on low incomes but also defended his plans to cut taxes for people earning more than £50,000 as “part of a package”.
He said public servants were being “dragged into the higher rates of taxation and that seems to me to be a legitimate subject for Conservatives to focus on and to want to fix as well”.
Mr Hunt told him: “I have spent my life trying to persuade people that we are not the party of the rich.
“If your very first tax cuts are for higher rate earners that’s the wrong message.”
Mr Johnson repeatedly challenged his rival to say if he had “no interest in alleviating the burden on the pressed middle” and “does he think people on middle incomes should be paying the higher rate of tax”.
As the pair argued on stage, Mr Hunt fired back: “Because Boris never answers the question we have got absolutely no idea what a Boris premiership would be about.”
On Brexit policy, Mr Hunt said getting a deal was “not about blind optimism” but a grasp of the details.
“We need leadership that is going to guide us through a big constitutional crisis and make a great success of Brexit, that means being honest with people about the challenges,” the Foreign Secretary said.
And he issued a stark warning about the prospect of suspending Parliament in order to force through a no-deal Brexit – something Mr Johnson would not rule out.
“When that has happened in the past, when Parliament has been shut down against its will, we actually had a civil war,” Mr Hunt said.
But Mr Johnson said: “I’m not going to take anything off the table, any more than I’m going to take no deal off the table.
“I think it’s absolutely bizarre at this stage in the negotiations for the UK – yet again – to be weakening its own position.”
The October 31 Brexit date also loomed large over the debate, with Mr Hunt challenging his rival to say if he would resign if the deadline was not met.
“My opponent is clearly not committed to coming out of the EU on October 31,” Mr Johnson said.
“I think it’s very, very important not to envisage any circumstances in which we would fail to come out of the EU on October 31.
“I don’t want to hold out to the EU the prospect that they encourage my resignation by refusing to agree a deal.”
Mr Hunt told him: “It’s not do or die, is it? It’s Boris in Number 10 that matters.”
The pair also clashed over their response to US President Donald Trump’s outrage over the leaked diplomatic messages from the UK ambassador in Washington Sir Kim Darroch which described the “inept” and “dysfunctional” White House.
Mr Johnson refused to guarantee he would keep Sir Kim in post if he became prime minister, but said: “It is vital that our civil service is not politicised by ministers leaking what they say. Whoever leaked that deserves to be eviscerated.”
Mr Hunt said: “I have made it clear if I am our next prime minister the ambassador in Washington stays because it is our decision.”
The Foreign Secretary insisted he was the right choice for Tory members because he could extend the party’s appeal beyond its core vote.
“In poll after poll I am the public’s preferred choice for prime minister because I appeal not just to those who already vote Conservative but those we need to win,” he said.
Mr Johnson promised to help the country “get back our mojo”, adding: “There is only one way to get this country off the hamster wheel of doom and that is to get Brexit done by October 31, to bring us all together with fantastic infrastructure, better education and by fighting crime.”