Conservative chairman Grant Shapps's speech this morning was designed to put the spotlight on "30 days of Labour chaos". But it is his comments on Ukip that have attracted most attention. Asked whether he could rule out a coalition with the party in the event of a hung parliament, he replied: "I can rule out - We are not going to do pacts and deals with Ukip". Shapps's unambiguous response contrasts with David Cameron's equivocation earlier this month ("I don’t want pacts or deals with anybody," was his carefully worded response). But Conservative sources have confirmed that this was not a slip of the tongue: they really are ruling out any arrangement with Ukip (be it a coalition or a confidence and supply agreement).

For several reasons, they are right to do so. First, the earlier refusal to rule out a deal with Ukip had the potential to inflict further damage on an already tarnished Conservative brand. While support for Nigel Farage's party has surged since 2010 (when it polled just three per cent), it remains toxic to many voters, not least those the Tories need to win over if they are ever to win a majority again. Polling has consistently shown, for instance, that ethnic minority voters - just 16 per cent of whom voted Conservative in 2010 - have an understandably negative view of Ukip. YouGov last year found that a quarter of current Conservative supporters wouldn’t vote for the party if it entered a pact with Ukip, with 5 per cent switching to Labour, 4 per cent to the Lib Dems and 16 per cent abstaining. In the seats that the Tories gained from Labour and the Lib Dems in 2010, and those they need to gain in 2015, the prospect of a Tory-Ukip deal threatened to repel centrist and liberal voters.

Second, the Tories have recently derided Labour over its refusal to rule out a deal with the SNP, seeking to portray Ed Miliband as "weak" and "desperate". It is far harder to level these charges if Labour can reply in turn that Cameron is preparing to shack up with Farage. The rejection of a pact enhances the rhetorical boast that the Tories are unremittingly focused on winning a majority.

Finally, the chance of Ukip holding the balance of power in a hung parliament is smaller than often implied. Even if it makes five gains (a credible result), the party will still have significantly fewer MPs than the Lib Dems and the SNP (who stand to make far greater gains), and perhaps fewer than the DUP.

The cost of not ruling out a deal with Ukip is almost certainly greater than the cost of doing so. The Tories are right to have spoken with conviction today.