With a few outstanding exceptions, such as Simon Jenkins or Henry Porter, most of Britain's columnists have a declared party preference or leaning. There is nothing wrong with this. On the contrary it can make their writing more interesting. But there comes a moment when loyalty may trump judgement, deflating their authority. For example, the Independent's Steve Richards is a Labour supporter quite capable of scathing comments on the party's tactics and problems but who sells the pass on the fundamental issue of its restructuring of the state. (He offered to debate this with me but never took up my agreement.)

The importance of this kind of issue is that once one recognises it as a matter of principle (liberty, nothing less!) a consequential breach may take place with all sorts of loss of access. When columnists often write against the latest folly of the party they favour this can strengthen their standing and enhance their importance for party insiders. It does not put at risk valued and hard-earned sources. The real test of character comes when they face an issue which their party establishment simply does not want to have raised at all.

Peter Oborne, writing mainly in his Daily Mail column, but with a strong line in TV reporting (see his latest harrowing Unreported World from Nigeria on Christian killing of Muslims), trumpets his conservatism without shame.

I shared a recent session with him at a marvellous weekend visit to Ullapool's Changin' Scotland which I hope to blog soon. Peter criticised me for my "hopelessly optimistic" revolutionary spirit and contrasted it to his careful, almost tender conservative ambitions to secure the best of our traditions and institutions. I gave as good as I got, but that's not the point of this post. Oborne told everyone to vote for Cameron, not hang him along with Labour, and he has talked of his sympathy and interest in what Cameron is seeking to achieve.

This makes his guest contribution to yesterday's Observer exceptionally important. I started reading Oborne after his 1999 book on Alastair Campbell. He asked how it could be that a red-top journalist could become the second most powerful person in Britain. He realised that something had gone wrong with our constitution and political culture for it to happen and it was not just a personal story or a matter of gossip (although that as well).

As a Conservative he began to realise he was witnessing the irrevocable ploughing up of his familiar landscape and he sought the conceptual tools necessary to measure this. He has thus a real grasp of the central role and importance for a potential prime minister of his spin doctor... for him, but more important for us and the country as a whole.

Which brings us to Andy Coulson. He is David Cameron's spin doctor, or to put it another way, he is much closer to Cameron the politician and much more important for the future of politics in Britain than Samantha. He ran the News of the World when the paper was involved in illegal bugging in what is a growing story uncovered especially by the Guardian's assiduous Nick Davies - who has just made further revelations and reports:

While Scotland Yard's public position remains that it did all that its resources and the law permitted, some police sources admit privately that they failed to fully investigate the case, that decisions may have been distorted by a fear of upsetting Rupert Murdoch's newspapers, and that it was "unfortunate" that the officer in charge of the inquiry, assistant commissioner Andy Hayman, subsequently left the police to work for News International as a columnist.

The emerging picture of the scale of Mulcaire's criminal activities is also potentially embarrassing for Conservative leader David Cameron's media adviser, Andy Coulson, who edited the paper at the time of the offences and who says he does not remember any illegal act.

In a towering column that anyone interested in British politics should read twice, Peter Oborne takes the issue on, clearly out of obligation rather than his usual glee. He concludes:

the case against Coulson today is considerably graver than the case against Campbell. As deputy editor and then editor of the News of the World, he was presiding over what can only be described as a flourishing criminal concern.

He sternly advises Cameron to have nothing more to do with his close aide and accomplice and sever all relations with him should he win the election. Otherwise, informed members of the public may come to the conclusion that Downing Street's media operation is being run by a one-time Murdoch employee whose victims are still being paid off to remain silent - creating a gigantic conflict of interest at the heart of government as issues of broadcasting, license fees and obligations of fair reporting are decided. Whether or not Fox News comes to the UK could be a consequence.

Oborne observes, the British media "operates under a system of omerta so strict that it would secure a nod of approbation from the heads of the big New York crime families". He could have included the BBC. This issue is as great as the expenses scandal.

Indeed, it is arguably greater. In an excoriating roar of anger over the crushing of parliamentary democracy in Britain, Graham Allan MP denounced the political tango of the executive and media. It is not an already broken parliament that is the problem,

"Greedy MPs" fill and sell newspapers and news programmes and free the media from the obligation to report big-picture stories like global recession, multi-billion bank bailouts, climate change, public finance.

Such stories require serious analysis, and all too frequently their reporting threatens the power and reputation of major corporate and other special interests - including owners of media themselves.

When government and media together are happy to collude in the continued collapse of parliament it is not surprising that there is so little public pressure for serious reform of our politics, for a new clearly defined role for parliament and MPs.

I published Allan's statement in full in OK in November. It is like the cry of a powerless witness unable to arrest and charge the real perpetrators behind the expenses scandal - who indeed are profiting from it. What he points the finger at is the deadly dance of proprietors and premiers. This is why the Coulson-Cameron relationship is so important and why it needs to be broken: we know who is pulling the strings.

Which brings me back to Oborne. This kind of frontal attack on both Cameron's electoral machine and the media's craven willingness to go along with it rises above insider reporting and gossip. It is an act of treason for a Tory, putting country before party. It is an outstanding piece of journalism. It should have been published in... The Times.