Take a Letter: Dear Dave, Nick, Ed and Nigel
Who says letter-writing is a lost art?
This morning, the leaders of the Labour Party, the Liberal Democrats and UKIP wrote identical letters to the Conservative leader to complain about his reluctance to take part in TV election debates unless the Greens are invited.
Then Green Party leader Natalie Bennett wrote to the first three leaders above with an "I agree with Dave" message about the need for her party to be included.
This afternoon, Plaid Cymru leader Leanne Wood got out her own stationery set to write to the same three leaders, challenging them to a debate in Wales.
Plaid say she explained that "unless a debate is held in Wales, then English and non-devolved matters will dominate the main television debates" (although the election is largely about non-devolved matters).
Ms Wood said: "At the moment there is a danger that people in Wales will be sidelined by the television debates. If the debates take place without Plaid Cymru and the SNP, then it is clear that the discussions will be dominated by England only matters, and devolved issues will not be taken into consideration.
"People in Wales have a right to scrutinise the parties that could have an impact on their lives after the election. With the prospect of a hung parliament becoming ever more likely, it is very possible that Plaid Cymru will hold the balance of power. The people of Wales should have the opportunity to hold Plaid Cymru to account.
"For this reason it is important that the leaders come to Wales and hold a debate here in Wales. Welsh voters should be given the chance to properly scrutinise the parties that will represent them after the UK general election."
At the moment, there is no proposal from the broadcasters to include the Greens in the debates so the prime minister has his get-out clause. But suppose the broadcasters decided to call David Cameron's bluff?
Professor John Curtice of Strathclyde University put forward this theory: "One of the things we should now take in mind and has certainly happened since the original proposals were published is that the Greens together with the SNP and Plaid have said that they would negotiate jointly in any post-election negotiations about the formation of a government.
"So one possible answer the broadcasters might say 'ok, we recognise the changed situation the SNP are doing rather well, the Greens are doing rather well, but you're going to act as one bloc after the election so we will indeed propose a fourth debate, perhaps in collaboration with the newspapers who are already trying to organise one, at which it will be up to the Greens, and Plaid and the SNP to decide which of their three leaders should represent that Westminster bloc."
The idea, he suggests would create more problems for David Cameron and make it much more difficult for him to say no. But as it depends on the three members of the bloc agreeing on a single campaign voice - not to mention a debate location - the prime minister may yet be off the hook.