The UK Independence Party will "completely change British politics" if it wins next year's European elections, head of policy Tim Aker has said.
He told the BBC that UKIP was "attracting former non-voters" by giving "people who have given up on the other parties a reason to turn out".
The party, which advocates the UK leaving the European Union, came second at the last European elections in 2009.
It was "absolutely" sure of a triumph next time around, Mr Aker said.
UKIP, which also calls for tighter controls on immigration, has seen its poll ratings improve markedly over the last few years.
It enjoyed its best-ever results in May's English council elections, gaining more than 140 seats.
But it has been criticised for relying too heavily on leader Nigel Farage to promote its policies.
There was also embarrassment this year as MEP Godfrey Bloom resigned from the party's group in the European Parliament after being criticised for joking that a group of female UKIP members who did not clean behind their fridges were "sluts".
Ahead of May's local elections, Conservative cabinet minister Ken Clarke dismissed UKIP as a "protest party" and a "collection of clowns".
However, Mr Aker said messages on the EU and immigration were getting through to disillusioned voters.
Asked on BBC Radio 4's World at One about a possible win in next May's European elections, he replied: "It would completely change British politics."
He added: "We are attracting former non-voters who haven't voted in 20 years... We are getting people who gave up on the other parties a reason to turn out."
This had left Labour and the Conservatives "panicking", Mr Aker said, as they had not dealt with "mass immigration and mass unemployment".
He said: "There's no sense of control. We can't control the numbers (of immigrants within the EU), so why take the risk?"
Mr Aker said UKIP was working to promote its other policies, which include allowing more grammar schools to be set up and support for shale gas mining.