Picking a bruising fight with the BBC and its defence correspondent Andrew Gilligan has not only reinforced mounting public unease about the accuracy of official statements. It has also cast fresh light on the hitherto shadowy remit of Downing Street's director of communications.
If I had been asked last September about Campbell's role in the publication of the intelligence services" assessment of Iraq's weapons of mass destruction, I would have said his responsibility was to advise the prime minister on how best to ensure that the dossier was presented to the news media and the public in the most effective manner.
I might have had my suspicions about internal manoeuvring within Downing Street, but I would have had nothing concrete to go on. Now we know differently. Campbell chaired the planning meeting to discuss the subjects to be included in the dossier. And, as we now know from the evidence he was forced to disclose word for word by the Foreign Affairs Select Committee, he tried to strengthen its scope and impact.
Equally revealing was Campbell's confirmation of the hole-in-the-corner route through which the second, so-called dodgy dossier reached the public domain. Disclosure to Parliament was only an afterthought. Once his staff in Number 10 had completed the February document, with its mix of plagiarised material and intelligence information, Campbell ordered that it should be handed out exclusively to six Sunday newspaper journalists travelling on Blair's plane to a meeting with George Bush.
So much for the prime minister's much-trumpeted assurance that he had turned his back on spin.
Although I felt the MPs failed to carry out the forensic cross-examination that Campbell-watchers like myself had been hoping for, it was the first time he had been held to account since his only other select committee appearance in 1998.
What emerged was a graphic illustration of the seemingly unchecked powers that Campbell can exercise on Blair's behalf. Campbell acknowledged that for "many months" he had been involved in discussions over the contents of the September dossier with the chairman of the joint intelligence committee John Scarlett (he described Scarlett as "a friend of mine"). It was Campbell who chaired the 9 September "planning meeting" for the dossier.
The next day he received the first draft of the dossier, which - contrary to Gilligan's report for the Today programme - Campbell maintained did include the warning that missiles could be deployed by Iraq within 45 minutes. Among the changes made to the dossier at his suggestion were a clearer explanation of the time it would take Iraq to develop nuclear weapons, and the inclusion in the executive summary of details about the range of Iraq's missiles.
The select committee was taken aback by the extent of Campbell's reach. Sir John Stanley, an armed forces minister in the Thatcher government, found it "very worrying" that Campbell had been so deeply involved and could submit drafting amendments. "Once you have a mix between those responsible for presentation and intelligence," Stanley said, "you risk calling into question the authenticity of intelligence reports& Politics ends up driving the intelligence."
Whenever I write about Campbell's behind-the-scenes work coordinating the presentation of Britain's role in recent US-led military offensives, I fall back on the praise he received for the assistance he gave in strengthening Nato's media operations centre in Brussels during the Kosovo conflict.
The supreme Allied commander in Europe general Wesley Clark paid Blair's spin doctor the ultimate compliment: "The right way to fight a propaganda offensive... is to tell the truth. But you need some smart people who can tell you what piece of the truth you are looking for."
Campbell's ability to exercise authority across Whitehall stems from the 1997 order in council that gave both him and the prime minister's chief of staff Jonathan Powell unprecedented powers as party political appointees to give instructions to civil servants.
This was a smash and grab raid on Britain's constitution: a party propagandist was allowed to take control of the flow of information from the state to the public. The impartiality of civil servants was weakened, especially in the government's information and communication service. This has been compounded by Blair's failure to honour his undertaking to introduce a new civil service act to strengthen codes of behaviour.
Within months of Labour taking office it was evident the influx of party appointees had few inhibitions about exploiting their new-found status as temporary civil servants. The prime minister's position within the cabinet and Whitehall had been strengthened immeasurably.
During the heady, early days of New Labour, I heard around Westminster of how the cabinet secretary Sir Robin Butler sought to reassure senior colleagues. He was convinced that Campbell, Powell et al were just a passing phenomenon; it would only be a matter of time before the special advisers were in retreat and the civil service regained the ground that had been lost.
Unlike previous Downing Street press secretaries, Campbell had the freedom to exercise the prime minister's authority across the divide between government and party. For example, none of his predecessors were able to carry out party duties inside Number 10; that restraint was soon brushed aside by Campbell.
In December 1999 Campbell used his Downing Street power base to orchestrate publicity for the high-profile defection to Labour of the former Tory MP Shaun Woodward. Butler's successor Sir Richard Wilson rejected Conservative complaints that Campbell had breached the special advisers" code, but he did suggest that in future the prime minister's press secretary should carry out his party business "at lunchtime or outside office hours". Needless to say Campbell had no compunction the following year about using the Number 10 press office to campaign against Ken Livingstone's bid for the Labour nomination for mayor of London.
Occasionally, Blair's spin machine has been required to disclose details about the shady work practices of the network of political advisers under Campbell's control. As these advisers" misdemeanours have multiplied so have the calls for action. It was Jo Moore's infamous instruction to government press officers to "bury bad news" that finally provided the catalyst for a determined push by the civil service hierarchy to rein in Campbell & Co.
A powerful line-up of the great and good is recommending new safeguards. Sir Nigel Wicks's Committee on Standards in Public Life has taken the toughest line. It recommended last April that a strict limit should be imposed on Campbell's executive powers. His authority to give instructions to civil servants should be limited to Downing Street, thus restricting his power to order around the government's 1,000 information officers.
Blair's response has been to set up a government information review group, under the chairmanship of Bob Phillis, chief executive of the Guardian Media Group. Submissions to the group, which hopes to report in September, reflect widespread unease about the lack of accountability. The Civil Service Commissioners, whose job it is to ensure the impartiality of the civil service, believe all special advisers should be required to speak on the record; Mike Granatt, head of profession for civil service information officers, wants all lobby briefings to be televised.
What I found so galling about Campbell's evidence to the select committee was his rant at BBC correspondents for their "constant denigration of politics and the political process". There was no point in his own journalistic career when he did not have "respect for... Parliament, the politicians and the work that they did - and that included politicians with whom [he] fundamentally disagreed".
None of the MPs bothered to ask Downing Street's director of communications if he had stuck to those same principles during his six years working for the prime minister. I would suggest there is ample evidence to show that he has encouraged the sidelining of Parliament, and has thus undermined the democratic process.
For my last book, The Control Freaks, I investigated the six occasions on which former speaker Betty Boothroyd rebuked ministers for allowing disclosure to the news media of announcements that should have been made first to Parliament. In each case the trail of responsibility led back directly to Campbell and the other political appointees in Number 10's strategic communications unit.
When questioned in 1998 by the Select Committee on Public Administration, Campbell told MPs his task was simply to ensure government policy was "presented in a coordinated and effective way". Perhaps the Phillis review group will have the courage to stand up to him and redefine his job description. I for one am not holding my breath.Nick Jones was a BBC correspondent for 30 years and is the author of Sultans of Spin (Orion, £7) and, most recently, The Control Freaks (Politico's, £8.99)