Conservative chairman Grant Shapps has ruled out a coalition Ukip in May - despite David Cameron's repeated refusal to make any such pledge.
The Tory MP said that there would be "no pacts or deals " with Nigel Farage's eurosceptics if the general election results in a hung parliament.
He also accused Labour of "chaos" after Ed Miliband failed to rule out a post-election alliance with the SNP.
But Ukip questioned whether Mr Shapps was the one who was "calling the shots".
Earlier this month the Prime Minister twice declined to say that he would not enter a coalition agreement with Mr Farage's party.
Opinion polls suggest that after May 7 there could be no single party with enough MPs to command an overall majority at Westminster.
The Tories are understood to be looking at a range of possible co-operation deals, including with Northern Ireland's DUP.
Pressed on the issue of Ukip Mr Cameron would only say: "I don't want pacts or deals with anybody".
But asked if he could rule out a coalition deal with Ukip, Mr Shapps said: "I can rule out - We are not going to do pacts and deals with Ukip."
He added that the Tories were focused on securing an overall majority, but acknowledged it was going to be "an incredibly close election".
He also accused Labour of "stopping at nothing" to secure power - including a potential coalition deal with the SNP.
Mr Shapps also suggested that any result other than Tory victory could result in "a large degree" of uncertainty.
"With the Conservatives, you may like it or not but you know what you are getting," he said.
In response Ukip's Patrick O'Flynn questioned whether Mr Shapps was able to make commitments on post-election deals.
"I'm wondering to what extent Grant Shapps is the man who really calls the shots in the Conservative party," Mr O'Flynn said.
"He's had a few reverses recently," he added.
He also said that he thought it "extremely unlikely" that Ukip, who have suggested the price of their co-operation could be an early referendum on the European Union, would go into coalition with the Tories.
Mr Shapps comments came as he launched a Tory document setting out what the party said was the story of the last month and Labour's "30 days of chaos".
But he was unable to give any details of how Conservatives would fund Mr Cameron's pledged £7 billion of income tax cuts.
The Tories have been accused of stoking fears about a Labour-SNP alliance in England.
A campaign poster - for use south of the border only - pictures Ed Miliband and Alex Salmond arm in arm standing outside No 10 alongside the slogan "your worst nightmare".
This week the party added Gerry Adams to the photograph when it was reported that there had been contact between Labour MPs and Sinn Fein.
Mr Shapps told Tory activists: ""We face a fork in the road - is it going to be a future with David Cameron or Ed Miliband? Recovery or ruin? Competence or chaos? For the first time in years, there's a really substantive economic divide between the two main parties, the two contenders to be prime minister."
"If Ed Miliband can't run his own campaign, I suggest he wouldn't be able to run this country either. We can't let this lot anywhere near Downing Street. Today there are less than 100 days until you get to make the choice between competence and chaos, until you choose whether Britain keeps going forward or turn back on the progress we've made. Less than 100 days before the most important election in a generation."
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