The Conservative Party faces defeat unless it can persuade voters it is compassionate, former Cabinet minister Iain Duncan Smith has warned.
The ex-work and pensions secretary, who resigned in March in protest at cuts to the disability benefits budget, said the party had become "casual" about the agenda.
Mr Duncan Smith said he had received letters of support from Tory MPs since he quit - and was even on speaking terms with Chancellor George Osborne, whose Budget triggered his dramatic exit from the Cabinet.
He told the Spectator the party needed to show it was about more than simply dealing with the public finances.
"The left are told by voters: yes, you may be incompetent but your heart's in the right place," he said. "Conservatives get the opposite: yes, you may be competent but we question your motives.
"When voters mark the ballot paper, they ask: are they good for me? That's the competence issue. But they also ask: are they good for my neighbour? If we have both boxes ticked, we get elected. But if we fail the good-for-my-neighbour test, we don't."
He added: "Winning the last election made us become casual about this ... It takes five minutes to lose this narrative, and a lifetime to get it back. This has been my concern over the past nine months; I've been worried that our reputation was being slowly chipped away and that we'd be left with the narrow message of caring only for finances.
"And being seen as favouring only those we regard as the wealth creators."
Mr Duncan Smith said he considered resigning after the election, where the Conservatives stood on a promise to cut £12 billion from the welfare budget "with no work done to see where the cuts were going to be found".
But he stayed in his post because "I felt I had to somehow ameliorate this process".
Mr Duncan Smith said that following his resignation "I thought there'd be a bit of frostiness" from MPs, but instead he had received messages of support - particularly from Conservatives elected in 2015.
"I've had a lot of letters saying: your agenda is what I believe in, it's what I campaigned on."
He said he had spoken to Mr Osborne since his resignation: "We bumped into each other in the lobby the other day; all very reasonable. We chatted a little bit.
"The funny thing is, this isn't personal with me. Nothing in politics is personal."
The former Tory leader, a prominent Brexit backer, said a vote to leave would allow a "completely new relationship" with the European Union.
"I never was for 'out', believe it or not. I actually thought it was possible to get a proper reform deal with the European Union and a semi-detached relationship," he said.
"Ultimately, that's where we'll end up. Vote Leave is about a completely new relationship with the European Union. It means we don't go round squealing and getting angry with them; they don't get fed up with us - and we have a really good, balanced relationship. One they'll be happy with.
"They can integrate as much as they like, even pursue single-state status - we won't be joining them."