It could be one of TV's better sitcoms.

Dave won't talk to Ed if Nigel is going to be around. Ed is dismayed because Natalie threatens to pop in. Locked out, Nicola and Leanne hammer at the door. Nick is agitated again. Meanwhile, the am-dram society's opening night looms.

Memories are short. Yesterday, critics spoke as though David Cameron is in breach of an ancient constitutional duty because, plainly, he sees no advantage in televised pre-election TV debates. In reality, it is the Prime Minister who embodies tradition.

In 1964, Harold Wilson was keen on a debate with Alec Douglas-Home. Two years later, installed in Downing Street, Mr Wilson saw no use in such an encounter when Ted Heath was his challenger.

Margaret Thatcher did not stoop to televised argy-bargy. John Major turned down the idea in 1992, but demanded a debate with Tony Blair in 1997. Mr Blair went from boasting that he would meet Mr Major "any place, any time" to failing to find a time or a place.

In 2010, Gordon Brown had no appetite for a debate with Mr Cameron and Nick Clegg. The then Prime Minister was given to believe he had no choice. Ironically, Mr Cameron did not profit greatly. To this day, Conservatives are convinced their lack of a majority can be traced to prime-time debates.

That belief forms part of the Prime Minister's motives. He would rather avoid debates entirely. As a poor second, he would tolerate Ukip's Nigel Farage at one event, but only - Mr Cameron cites fairness, improbably - if Natalie Bennett is there to represent the Greens. In other words, if the Prime Minister is to be threatened from the right, Ed Miliband and Labour should face a threat to their left.

Everyone accuses everyone else of self-interest. Everyone is right. Yesterday, Mr Miliband, Mr Clegg and Mr Farage despatched a letter to broadcasters insisting that the Prime Minister should be represented by an empty podium if he continues to drag his heels. But none of the three wants the Greens involved. They certainly offer no encouragement to Nicola Sturgeon and the Scottish National Party, or to Leanne Wood of Plaid Cymru.

This is a mess, one concocted by Westminster, broadcasters and Ofcom as they struggle to contend with a fragmenting political order. What are still called big parties have shrunk. In the case of Mr Clegg and the Liberal Democrats, only past glories remain. At least one quarter of UK voters have withdrawn support from the usual triumvirate. If TV debates matter in a modern democracy, the fact should be recognised. After all, other countries manage multi-party debates without fuss.

The SNP are wild cards in this poker game. Their membership surge makes them easily the third-largest party in the UK, but the regulator and broadcasters take no account of the fact because the nationalists have no candidates beyond Scotland. On the other hand, polls say the SNP could influence the shape of the next UK government. Which criterion applies?

Like a good sit-com, this one will run and run. If broadcasters stick to their guns, however, and insist on involving only the old "main" parties (with a walk-on part for Mr Farage), all bold claims made for the importance of TV debates will begin to seem flimsy indeed.