Ukip's Steven Woolfe launched his bid to succeed Nigel Farage as party leader, promising to "ruthlessly" go after Labour voters.
Mr Woolfe said Ukip has "won the argument" for managed immigration and promised to drive a new focus on improving social mobility.
The MEP said he would professionalise the party but insisted it would remain the self-styled "people's army" Mr Farage created.
Launching his bid for the leadership in central London, he said: “To old Labour voters let down by your party, I say Ukip will give you a voice. To the majority of Tory voters who voted Leave but have been lumbered with continuity Cameron under Theresa May, I say Ukip will welcome you.
"We must ruthlessly go after Labour seats in the North and the Midlands.
“And we must say to the 17.4 million people who voted Leave on June 23 - Ukip speaks for you. We are your new home.”
Mr Woolfe said he wanted to tackle social mobility, a subject discussed "but never dealt with effectively", in an effort to win over voters left behind by Labour and the Tories.
Insisting Ukip could be a "force for good", he said: "Millions of people up and down our nation have no say in this country as this country has not invested in them.
"It has left them on lower wages with poor schools, long waiting lists at hospitals, and they have been called stupid, racist and ignorant when they express valid concerns about the direction we are heading."
Mr Woolfe said Labour and the Tories now fear the electoral threat Ukip as he launched a blistering attack on Labour under Jeremy Corbyn.
"The Labour Party once championed those workers, but decades of being run by champagne socialists, metropolitan, out-of-touch, liberal elitists from their multimillion pound estates and houses in Islington town has seen Labour abandoned its working class base.
"It is now even imploding in front of our eyes.
"Labour MPs - bar a few notable and honourable exceptions - voted in their droves to remain in an anti-democratic, corporatist European cartel, totally at odds to the history of the Labour Party whilst most of the die hard supporters voted to Leave.
"Never before has the Labour Party been so distant from the views of their once loyal base."
He added: "It's our job to speak for those who have been left behind, we must provide a voice for the millions who have been deserted by the Tories and the Labour Party."
Fellow MEP Jonathan Arnott is the only other Ukip member to announce his candidacy for the leadership.
The race is already proving controversial because the Eurosceptic party's ruling National Executive Committee (NEC) said all candidates must have been a member for at least five years.
It means Ukip's only MP, Douglas Carswell, ex-MP Mark Reckless and prominent former spokeswoman Suzanne Evans - who is temporarily suspended from the party, will not be able to enter the contest.
Candidates must pay a £5,000 deposit to enter the contest, which will be refunded if they secure at least 5% of the vote.
Nominations, which need to be backed by a proposer and 50 supporters from at least 10 branches, opened on Monday and will close on July 31.
During August, regional hustings will be held before the count and declarations are held on September 15.