David Cameron has said he does not think Ukip will win any seats in the General Election.
He told ITV News he would not lead a government unless he could deliver it.
Asked if they meant he might have to go looking for votes from Ukip, he said: "I think voting UKIP is in danger of letting a Labour MP in."
Union leaders have attacked the Prime Minister for likening himself to a firefighter in dealing with the "mess" left by the previous Labour government.
David Cameron told a meeting in Wetherby, Yorkshire, that he sometimes felt like a firefighter, putting out a blaze in a building which had been started by Ed Miliband.
Matt Wrack, general secretary of the Fire Brigades Union, said: "Firefighters will be appalled that David Cameron has the gall to draw comparisons between himself and the lifesaving service firefighters provide.
"Under Cameron's leadership the fire and rescue service has been decimated by funding cuts and the pensions of thousands of firefighters stolen.
"Cameron has put the public at risk as emergency 999 response times continue to rise and fire engines are now frequently understaffed.
"If Cameron really thought like a firefighter then he would recognise the funding crisis hitting the fire and rescue service and work to resolve it."
The union has been embroiled in a bitter row with the Government over controversial changes to their pensions, which has sparked a series of strikes.
The FBU said over 5,000 frontline firefighter jobs have been lost and more than 40 fire stations have closed over the past five years.
David Cameron appeared to make a slip up when he referred to the election as a 'career-defining moment' before quickly correcting himself.Read the full story ›
David Cameron said having an EU referendum is a "red line" when it comes to creating a coalition government
"I will not lead a government that does not deliver that pledge" of holding a referendum, Cameron reiterated.
"If you get me as Prime Minister you get that chance to have that in-out referendum," he added.
George Osborne has tweeted that David Cameron has given the "performance of a lifetime" during his Question Time appearance.
David Cameron just gave the performance of a lifetime - strong, personal and profoundly optimistic about Britain's future
David Cameron told the Question Time audience that Britain has a benefits system that "skews it in favour of people coming to work here".
"That needs to change - now that needs change in Europe and it needs a Prime Minister that's prepared to get change in Europe," he added.
Cameron said he wants to see net immigration from outside the EU to "come down to under 100,000".
David Cameron has said the Conservatives "can finish the job without putting up people's taxes".
"If you want a government that finds more efficiencies in government spending, that goes on reforming welfare and doesn't put up taxes for working people, indeed cuts taxes ... that's me," Cameron said.
David Cameron has told leaders' Question Time his government had increased child tax credits rather than cut them as Danny Alexander suggested.
Cameron said he had rejected the idea of cutting child tax benefits, but stressed that he wants to keep reforming the welfare system.
Danny Alexander has accused the Conservatives of being bent on "slashing support for families" as he went public with Tory proposals for an extra £8 billion of welfare cuts that were vetoed by the Liberal Democrats.
The Tories have dismissed the claim as "desperate stuff" from the Lib Dems, and insisted the proposals were "definitely not our policy".
ITV News Political Correspondent Emily Morgan reports from the Lib Dem campaign trail: