David Cameron
David Cameron has underpinned his pledge on an EU referendumReuters

David Cameron's latest attempt to stop Tories defecting to Ukip by offering new pledges on his promised "in-out" referendum on EU membership, have already been filed under the heading "statements of the blindingly obvious".

The prime minister's words, in a phone-in with party activists, amount to a repeat of his previous promise to hold the vote in 2017, but with the added emphasis that he really, really, really means it.

The first problem is with his statement: "I would not be prime minister of a government unless we could carry out our pledge of an in-out referendum."

It appears Cameron is saying that he would step down as prime minister after the 2015 general election if he was stopped from delivering on his promise.

But it begs the question that, if he was elected with an outright majority in 2015, what could possibly stop him from holding such a referendum other than his own change of mind?

It would not be Tory MPs, who are overwhelmingly behind the idea. And the Lords would certainly not attempt to block what would have been a clear manifesto promise.

So, as far as that part of his pledge goes, nothing has really changed.

Then there is the other part of his pledge that "this is not something I would ever barter or give away", suggesting it would be a red line in any negotiations with the Liberal Democrats over another coalition government.

But, if Cameron fails to win an outright majority for the second time in a row, what are the chances he would survive Tory party anger and be allowed to remain as leader and in a position to do any bartering at all?

And, in any case, it had always been accepted the referendum would be a red line in any negotiations, if Cameron was still in a position to do deals. Any attempt to abandon it in order to secure a coalition would also spark a backbench Tory revolt.

His already fractious Eurosceptic hard-liners would almost certainly demand his head in the hope of getting a leader more to their liking and who they could trust to deliver on Europe, even if that meant failing to form a government and forcing another general election.

Potentially, in those circumstances, Cameron might be able to remain as leader if he promised to immediately call a second election specifically aimed at securing a mandate on the referendum pledge

But these scenarios were already likely and his latest comments have not changed that either. Neither have the positions of Labour or the Liberal Democrats changed and it still remains the case that the Tory party is the only one with the possibility of winning power in 2015 that is offering that referendum

However, that fact has not stopped voters surging towards Ukip for the EU elections next month, and it is that poll Cameron was really focusing on in his attempt to win some of them back with the strength of his commitment, and limit the looming damage.

What happens when voters have expressed their opposition to the big three parties next month and start looking towards electing a government has always been another matter. But, once again, nothing Cameron said changed those calculations either.