Nick Clegg yesterday issued a direct challenge to David Cameron to publicly rule out a post-election alliance with Ukip, telling him not to treat voters as if they are “stupid” by maintaining the Tories are on course to a majority.

The Liberal Democrat leader said he would not join a government with the SNP or Ukip and called on the Prime Minister to do the same with Nigel Farage’s party.

In an appearance on BBC1’s Andrew Marr Show, Cameron declined to rule out any post-election deal with Ukip if there was a hung Parliament.

At a campaign rally in Portsmouth’s Spinnaker Tower, Clegg said: “This morning, David Cameron refused to rule out on five separate occasions entering into an alliance with Ukip.”

He added: “David Cameron: don’t treat people as if they are stupid. You know, everybody knows, that you and the Conservative Party are not going to win a majority at this election. I have ruled out an alliance with the SNP, I have ruled out an alliance with Ukip, why won’t you do the same?”

Addressing the gathering in the tower at Portsmouth Harbour, Clegg said only the Lib Dems could act as a “barrier” to Ukip or the SNP being in power. He said the Tories’ “ideological approach” would result in £60 billion of cuts to public services.

“The reason why it is so extreme is because they won’t ask their friends in big business and big houses, the very wealthiest, to pay a single extra penny through the tax system to balance the books.

“They have made an ideological choice to cut, cut, cut way beyond what is necessary and to single out only one section of the population – the working age poor – to pick up the tab for the mistakes made by the banks.”

Clegg continued: “Liberal Democrat MPs are the only MPs in the House of Commons who can act as a barrier between Alex Salmond and Nigel Farage and the doors of Number 10.” He repeated his warning that Cameron could be a “hostage” to Blukip – an alliance of the Tory right, Ukip and the DUP – if he tried to govern without a majority in the Commons.

Speaking to reporters he said: “I think the Conservatives are getting away with, if I can use the pun, blue murder by claiming that if you vote for the Conservatives that gives a guarantee of stability and continuity against the risk of a Miliband/Salmond government.

“The real risk also is the other way – if you vote for the Conservatives you will get Blukip – you will get an alliance of real hardline right-wingers in the Conservative Party who have been very clear they want to enter into an alliance with Ukip, Ukip have said that as well.”

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