The Conservative leadership candidates do not reflect the eurosceptic beliefs of the bulk of the party's activists, says the UK Independence Party.
Speaking at the party's annual conference UKIP leader Roger Knapman also accused frontrunner David Davis of being hypocritical over Europe.
He also tried to play down election losses in May 2005 and the departure of some of its senior supporters.
Earlier this week the party's chairman Petrina Holdsworth resigned.
'Fair weather friends'
UKIP failed to make a breakthrough at the 2005 general election, with most of its candidates losing their deposits. The party won twelve seats in the European Parliament in the previous year.
Mr Knapman, a former Tory MP, told delegates: "I'm pleased to see you here. You're the engine room the people who stick at it and I've no time for the Jerimiahs or the fair weather friends.
"A few have gone, so what? What were they worth anyway? Let's have the people who are really committed to the party and to the nation. Let's increase now our membership again."
He said UKIP's membership had "only gone down by a small amount" and was starting to increase again.
The leader, a former Tory MP, said UKIP should remain clear about its aims, despite recent difficulties.
"We will face setbacks, in fact almost on a daily basis sometimes it seems. There will be times when will falter but we must never, never lose sight of our objective.
"It is only UKIP that is clear in its objective, which is to regain our independence and sovereignty. It doesn't take a lot to say 'no'."
Criticising the contenders for the Conservative Party leadership, he added that David Davis was not facing up to his own past actions.
"Perhaps what the Conservative Party membership ought to know is that it was David Davis that bullied and arm-twisted the Maastricht Bill through Parliament from his position in the then whips office.
"If this is suddenly the best Eurosceptic that they can offer, then it is a real conversion."
He also criticised candidate David Cameron for "his silence on the subject" (of the EU) and describe Dr Liam Fox as "astonishingly naive".
UKIP, left without its most high-profile campaigner Robert Kilroy-Silk, lost its deposits in at least 451 seats at the last general election.
While UKIP MEP Nigel Farage acknowledged the party's fortunes had ebbed since the European elections in 2004, he said it was largely because issues like the euro and the EU constitution had slipped down the political agenda.
He told BBC Radio 4's Today programme: "Our job now is to show people they are implementing the constitution anyway, we have had Turkey talking about joining the EU and integration is pushing ahead quickly.
Mr Farage played down the significance of the Ms Holdsworth's resignation on Tuesday.
"This job is an unpaid job. It costs a fortune to do. You are on call seven days a week and all you get is aggravation," said Mr Farage.