The Greens have unveiled a new poster, saying: 'What are you afraid of, boys?', suggesting that the other leaders are scared of debating the Greens.
The Greens have not been invited to participate in the TV debates. At the moment, David Cameron is refusing to participate without the Greens. Whether that decision is about diversity or selfishness is up for debate.
So what are the 'boys' afraid of?
It's obvious why the Labour leader does not want to debate Natalie Bennett. The Greens are a Party on the left and as Labour have moved further from the left in recent years (particularly under Tony Blair), Ed Miliband is afraid that giving the Greens the opportunity to challenge Labour in a debate could persuade some Labour voters to jump ship to the Greens.
Additionally, if the Greens are included then there could be a case for including the SNP. If the SNP joined in on the debates then that could potentially help the SNP get more votes from Labour north of the border.
In another time when UKIP were not so big, it would be likely that the party would advocate for including the Greens as that would boost their own case for being included. But now, UKIP have been given 'major party' status by OfCom. Nonetheless, the reason Farage will not want Bennett included is the loss of his outsider status. If the debates go ahead with just four leaders, Farage will benefit from being the outsider as he does not lead the opposition nor is he in the government. Including Natalie Bennett would result in two outsiders - two non-MPs - which could weaken Farage's dominance on the outside stage.
There are likely two reasons why Nick Clegg is likely scared of having Natalie Bennett in the debates. The first is that in some polls the Greens are catching up with the Lib Dems. For example, in the Ashcroft poll already mentioned.
And secondly, a significant proportion of voters who chose the Lib Dems in 2010 plan to vote Green in May. YouGov's recent poll, for the Sunday Times, which puts the two parties neck and neck on 7%, suggests that 14% of those who voted Lib Dem in 2010 are planning to vote Green in May.
The full results of the poll can be found here.
The Prime Minister is not afraid of the debating the Greens. But he is arguably scared of the debates themselves. In fact, saying that the Greens should be included can benefit the Prime Minister. If debates do not happen, then he does not have to debate. If he gets his way - and the Greens are included - then the Greens can attack from the left, and that could help take votes from Labour, which could ultimately help the Tories.
Will the debates happen?
Who knows, but William Hill have odds for David Cameron participating in a debate with four or more leaders at 1/2. The odds of him not participating in them are 6/4.