In the first place, Mr Mandelson isn't just any minister. He is the key member of the team that created "New Labour", or at least the Blairite revolution as defined in the world of sound-bites, focus groups and rhetoric. In his absence, Mr Blair has not only lost a close supporter but one whose weight - and access to the Prime Minister - shaped the balance of power in the Government. Without him, other forces are bound to move to fill the vacuum, in an air already poisoned by accusations that his downfall was plotted in the court of Chancellor Gordon Brown. As history has shown repeatedly, the way to wound a king is always by hurting the closest adviser.
And as Mr Brown moves in from one direction, his opponents are closing in from others. The combination of John Prescott and Robin Cook, which we report today, is not just an alliance of the previously marginalised in the Labour Cabinet. It could mark the resurgence of that significant group in Labour which has never been fully committed to Mr Blair's cherished vision.
Which brings in the other point. "Project New Labour" was fine as a slogan for opposition, a standard by which the forces of the left could present a more reassuring and modern face to the voting public. But Labour isn't in opposition anymore. It is in government - 18 months into government, indeed. And it will be defined by what it does with power.
In that sense the Prime Minister could understandably argue in his radio interview yesterday that all this concentration on personalities is out of proportion. It is education, the economy and health which matter and those will have to be dealt with whether or not Mandelson or Robinson are present in government. But appearances do matter today.
Peter Mandelson didn't resign because he broke the rules of ministerial behaviour. He went because, after a day of trying to justify his behaviour, it became apparent that the press and the public weren't going to wear it. So as his rivals feud and bicker like so many medieval barons, the man who most understood New Labour is in exile.
Now comes the great test for the Government, the one on which all governments ultimately succeed or fail: namely, the test of competence. Far from this being just a case of a minister who did wrong, who resigned and "it's over", as Mr Blair argued yesterday, his problems of prime ministership are only just beginning. Can we suggest a biography of Harold Wilson as his holiday reading in the Seychelles?