One of the dangers of Nick Clegg's decision to debate Nigel Farage was always that it would encourage David Cameron and Ed Miliband to host their own head-to-head (as I previously noted  here). Both the Tories and Labour see potential benefits in a one-on-one contest between the two men fighting to become prime minister. Aware that Cameron outpolls both his party and Miliband, the Conservative have long intended to frame the election as a presidential battle ("do you want David Cameron or Ed Miliband as your prime minister?") and a debate would be the ideal way to amplify this impression. Conservative whip Greg Hands gave the game away when he tweeted  during the German leaders' debate: "Interesting that German TV debate only has the leaders of the two parties who could conceivably be the Chancellor. No FDP, Greens, etc".
Although Labour is pushing for a repeat of the 333 format (three debates between three leaders over three weeks), some in the party believe that a Cameron vs. Miliband contest would help the party to retain the crucial group of Lib Dem defectors.
With Cameron still refusing to commit to the debates, and other Tories continuing to float the idea of excluding Clegg, the Deputy PM has publicly intervened, telling the FT : "I struggle to think of even half a respectable excuse the Conservatives could come up with to deny the British people the right to see the party leaders measuring up against each other in a leaders’ debate.
"Ed Miliband and I said we’ll sign up on the dotted line, and repeat the format of last time. People found it a useful innovation and I think the Conservatives shouldn’t run away from having the kind of debate that we had last time."
An aide suggests that the exclusion of Clegg would breach impartiality rules and refuses to rule out legal action to block a Cameron-Miliband debate. The aide has in mind the Ofcom rules  which classify Labour, the Tories and the Lib Dems as "major parties" for the purpose of general elections (the regulator will need to decide whether to add Ukip, as it has done in the case of the European election ) and require broadcasters to give equal treatment to them. But crucially, these only apply during an election campaign, not outside of it. One suggestion made by Cameron is that any debates (which he complained "sucked the life out of" the 2010 campaign) could take place before the dissolution of parliament. Were that to happen, Clegg could struggle to avoid being left out.