The Ukip leader said his party “could never” do a deal with Ed Miliband because of his position on the EU referendum.
He told broadcasters: “He has said he will not give the British people a referendum on the great European question.”
But he added: “Mr Cameron at least has been forced into promising that, and so after the election he is somebody we can sit down and talk to.”
The remark will increase pressure on the Tories to rule out a coalition with Ukip if they fail to secure a majority on May 7.
Labour has urged the prime minister to “come clean” over whether he would strike a post-election deal with the party.
Shadow health secretary Andy Burnham has written to Mr Cameron demanding that he explain what his position would be if Ukip held the balance of power.
Both Mr Cameron and Chancellor George Osborne have failed to rule out a pact.
But Conservative defence secretary Michael Fallon excluded the possibility of “working with” Ukip in the event of a hung parliament.
He was initially asked on BBC News why the party would not rule out a coalition with Ukip, to which he replied: “Because we want a majority government.”
But asked whether the Tories would be prepared to work with Ukip if necessary, he said: “No, We have already said we are going for a majority government. We are not in the business of doing deals.”
He was then asked to imagine the scenario where no one party can form a majority government and in that instance whether the Conservatives would work with Ukip.
The Tory minister replied: “No. Look, it is our job to put over our policies.”
Asked to clarify if that was a no, he said: “That was a no. We are pointing out the dangers of going for a coalition. If you vote for the Conservatives, you will have David Cameron with a proper working majority ... We are working flat out for a proper majority.”
Meanwhile, Nicola Sturgeon denied handing the keys to 10 Downing Street to David Cameron with her commanding performance during the leaders’ debate.
The Scottish First Minister said that if the election delivered a landslide for the SNP and Labour made a deal with the nationalists then together they actually lock the Tories out of number 10.
Ms Sturgeon is widely considered the winner of the high-stakes, seven-way showdown, although snap polls gave no clear victor, particularly in Scotland where she is being praised for delivering a “Sturgeon surge”.
Labour accused the Conservatives of “bigging up” Ms Sturgeon in an attempt to return Mr Cameron to power.
The SNP is forecast to deliver a significant blow to Labour in Scotland, all but wiping out the number of the party’s MPs. This could be crucial at a time when neither Labour nor the Conservatives are expected to win a majority on 7 May.