Nigel Farage says other parties 'fear' UKIP
Nigel Farage has said the Conservatives and Labour both "fear" a UKIP breakthrough at May's general election giving it the balance of power.
In his first major speech of 2015, Mr Farage said he believed no party would secure a majority after May's poll.
But UKIP would not "prop up" a government led by another party unless it offers an "immediate" EU referendum.
The UKIP leader also claimed his party was now the only "truly national political party" in the UK.
He dismissed the Conservatives as a "regional party for the South of England" and Labour as a similar party for the North. saying UKIP, by contrast, was "the challenger in virtually every parliamentary seat from Birmingham up to Hadrian's Wall".
He said UKIP was also "beginning now to dig quite deep into some of the ethnic community vote".
Speaking at a cinema in Canvey Island, Essex, he said UKIP would not engage in "dirty politics" as its opponents "turned nasty" during the election campaign.
Analysis, by political correspondent Robin Brant
This was a soft campaign launch but it was an important speech from Nigel Farage.
It was heavy on the rhetoric and made no attempt to hide the fact that UKIP is a party tugging at the heart strings, perhaps learning a lesson about the power of emotion from the (ultimately unsuccessful) Yes campaign in Scotland.
The word "believe" was uttered dozens of times in Mr Farage's short speech; believe in Britain, believe in change.
But it was light on weighty policy. Everything mentioned from the podium, in a cinema by the Thames Estuary, has already been announced and it was mostly low level "retail" policies, such as ending car park charges in NHS hospitals.
Nigel Farage repeated two key things: UKIP won't go into a coalition with any party after 7 May's general election, if it gains more MPs, and their big negotiating demand - their only one - is an "immediate" EU referendum.
But Suzanne Evans, who is writing the party's manifesto, refused to answer my question about their deficit reduction plan. And on the NHS - an increasingly significant issue for UKIP - Mr Farage wouldn't say if he believed the current way of funding the health service was sustainable.. All he would say was there can't be any cuts.
David Cameron, Ed Miliband and Nick Clegg are also staging campaign events on Thursday, the final day before Parliament breaks up for a 10-day recess.
Mr Farage, who is hoping to be elected to Westminster as MP for South Thanet, went on to suggest that the two largest parties were failing to "cut through" to the public.
"Out there, beyond Westminster, the small businessmen, the backbone of the country, have become victims of a modern form of corporatism.
"We are the only party standing up for the little man."
Writing in the Daily Telegraph, Mr Farage suggests that campaigning this year has been "incredibly dull so far", with Labour's focus on the NHS and the Conservatives' prioritisation of the economy "pretty predictable".
The UKIP leader has said his party's main policies, including leaving the EU, taking all those on the minimum wage out of tax, ditching HS2 and scrapping tuition fees for science, maths, medicine and engineering degrees, have a wide appeal across the country.
Addressing UKIP's policy on the NHS, Mr Farage says it would inject £3bn into the health service but would change the way the institution operated, prioritising research and innovation over middle management "to make it work for those who need it".
His remarks come amid continuing debate in the party over how to meet the rising demand for healthcare at a time of continuing strain on the public finances.
Labour accused UKIP of following a Conservative agenda on the NHS and the economy.
Shadow Health Minister Jon Trickett said: "No single speech is going to hide UKIP and Nigel Farage's long-held beliefs on increasing NHS privatisation and further tax breaks for those at the top.
"The truth is UKIP are a party of Tory people, Tory policies and Tory money: they are more Tory than the Tories."