Suggestions that David Cameron has rediscovered the joys of environmentalism can probably be discounted.

The Prime Minister reputed to have told his ministers to "get rid of all the green cr*p" in the winter of 2013 has not suddenly become a friend to parties favouring that colour.

This week, nevertheless, Mr Cameron told broadcasters it would be jolly unfair, in his "very strong opinion", to stage pre-election TV debates if "the Greens" were excluded. Such was his indignation, he was ready to decline an invitation to participate in any spot-the-loser shows. Some things were immediately obvious, others far from clear.

One obvious thing was that Mr Cameron wants to avoid TV debates of any description if he can possibly help it. From his perspective, this is just common sense. These affairs favour the challenger - or challengers - not a Prime Minister with everything to lose. Until Gordon Brown agreed to participate as a last throw of the dice in 2010, his predecessors had refused to stoop to TV talent shows.

The second thing, slightly less obvious, counted as a tactic rather than a strategy for Mr Cameron. It amounted to saying that if there was even a remote risk of his becoming a punch bag for Ukip's Nigel Farage, Ed Miliband could have a Green sitting on his shoulder like Jiminy Cricket, speaking all those leftish words of conscience the Labour leader dare not utter. Predictably enough, Labour are furious.

Once those bits of mere politics are understood, however, things become more complicated. Party allegiances aside, they force us to ask what is really going on in the staging of pre-election debates, what influence they have on the democratic process, and how they can possibly work fairly in a multi-party era.

Mr Cameron was speaking after the launch of a faintly risible Ofcom "consultation" in which the regulator suggested the Greens are not a major party. In contrast, it was said Ukip might just qualify, albeit only in parts of the United Kingdom. By sheer coincidence, this was perfectly convenient for the broadcasters - the BBC, ITV, Sky and Channel 4 - who want three debates involving combinations of Mr Cameron, Mr Miliband, Nick Clegg and Nigel Farage.

For Natalie Bennett, leader of the Green Party, more is at stake than five minutes of fame in debate and the chance to make Labour uneasy. If Ofcom confirms its draft findings, she loses out on two party election broadcasts on participating TV and radio channels (the BBC makes its own decisions by "relevant nation"). Those inclined to vote Green would also be denied the chance to be taken seriously, for a change, by the broadcasters.

Ms Bennett's complaints and her party's threat of legal action are only the half of it. In attempting to assess who is major and who is, by definition, minor, Ofcom has managed only to show how badly broadcasting and its regulator reflect modern political realities, if they even bother to make the attempt. The proposed TV debates are equally revealing: they were a bad joke in 2010; they will be preposterous in 2015. They will amount, if you like, to a misrepresentation of the people.

Whatever his motives, Mr Cameron made one fair point. He said: "The Greens have a member of parliament, they beat the Liberal Democrats in the last national election - the European Elections - so I don't see how you can have Ukip and not the Greens." Ukip have only a pair of Tory defectors and a poll rating - according to a YouGov survey published on 8 January - of 13 per cent. What's more, even Ofcom concedes - it could hardly claim otherwise - that Mr Farage's faction count as "major" only in England and Wales.

Will that fact deter broadcasters from treating the voters of Scotland to an evening's entertainment from Ukip? Not a bit of it. No doubt STV and the BBC's branch office will come up with debates of their own, but that doesn't answer the question. The tension between major and minor, "regional" and "national", will be at the heart of the 2015 election. The fact, to put it delicately, is being obscured by Ofcom and by the broadcasters, separately and together.

The old "main" parties are less secure than ever. According to polls, one third of voters decline to support the Tories or Labour. Mr Clegg's LibDems are being matched in those same polls by the Greens. Yet Mr Farage alone is being allowed into the studio simply because his party has made some inroads in England and Wales, and because broadcasters persist in believing he provides good telly.

Unsurprisingly, not for the first time, the Scottish National Party have been all over this. They fully support the claims of Ms Bennett, but wonder about Mr Cameron's arithmetic. He's standing up for a party with one MP? The SNP have half a dozen and every confidence - if opinion polls and recent performances are a guide - to secure many more seats. Ofcom cannot tell the broadcasters who to invite to their debates, but parallel choices are being made. Strange choices they are, too.

The obvious case involves the SNP. The regulator defines them as "major" in Scotland, but nowhere else. On the face of it, that makes sense. Why on earth should a constituent in Kent be favoured with a party broadcast from people for whom she cannot vote? But what if, as polls suggest, that party are about to form the third-largest grouping in the woman of Kent's parliament? Isn't she entitled to hear what they stand for?

The point ought to be acute for broadcasters desperate to stage debates. The 2015 campaign is already notoriously unpredictable. No one knows how the cards will fall. But the possible influence of the SNP on the composition of the next Westminster government is central to the chatter down by the Thames. It's a fact of political life. Yet those planning debates for all the peoples of the UK mean simply to ignore that fact.

This is tantamount to interference in the political process. If the broadcasters cannot accommodate all relevant parties in their formats, why stage these personality contests? Too many people cast their votes last time around just because they'd heard X sounding quite good compared with Y for five minutes. If relevant voices are excluded, even TV's garish version of a hustings loses all justification.

Back at Ofcom, opinion polls remain a factor in the regulator's decisions. The LibDems, says YouGov, had only 8 per cent support in its latest survey, just one ahead of the Greens, yet Mr Clegg's party are still nominated as one of only three major UK parties. Few doubt the LibDems will lose a majority of their 56 MPs in May, yet still they are taken under Ofcom's wing.

This has nothing to do with political reality. It is not - that small detail - even slightly fair. Millions are being excluded by the regulator and broadcasters alike. And the fact leaves millions with the old, familiar impression that Establishment politics in the UK will forever be a good, old-fashioned carve-up.