Deputy Prime Minister Nick Clegg has launched a bitter assault on the UK Independence Party, accusing it of peddling "dangerous fantasies" that Britain's problems could all be solved by leaving the European Union.
With the opinion polls suggesting Ukip will top the poll in the European elections later this month, the Liberal Democrat leader said that his party was now the only one prepared to stand up to them.
Launching the Lib Dem campaign for the European and English local council elections, Mr Clegg sought to brace his party for heavy losses, saying voting was taking place in a "totally different context" from the last time the seats were contested when the Lib Dems were still in opposition.
Speaking in the incongruous surroundings of the Ministry of Sound nightclub in south London, he acknowledged that Ukip was now picking up the "protest votes" that had in the past gone to the Lib Dems when they were in opposition.
"Bluntly, we are not the protest vote any more. We are not the 'none of the above' party any more. We're not the 'we don't like the world, let's get off' party any more it," he said.
"Ukip is now the protest party in British politics, scaring everybody witless, indulging in dangerous fantasies, claiming somehow miraculously every problem on the face of our planet would be lifted from our shoulders if only we just left the European Union.
"Well, let them continue with those dangerous fantasies. We will continue to argue for what we stand for."
Weekend opinion polls showed the Lib Dems trailing in fourth place in the European election on 10% behind Ukip on 29%, Labour on 26% and the Conservatives on 23%.
Mr Clegg sought to make a virtue of his party's willingness to champion "unfashionable" causes such as the environment and international aid which he said had been abandoned by the other parties.
"We're not asking you to vote for us to just stick two fingers up against the other two parties. We're asking you to vote for something," he said.
"We are the only party in these elections that have the courage to stand up to Ukip, the only party relied upon to rein in the Conservatives, the only party to undo the damage done by Labour to our economy."
Voters go to the polls for the English council and European Parliament elections on May 22.