Behind the lies, leaks and smears, how can one get even close to the truth? The classical method is to ask Cicero's penetrating question: cui bono? Who benefits?

In the case of Damian McBride's notorious e-mails, we know what they contained and we know who wrote them. There is no question about the basic facts. The e-mails proposed a smear campaign against senior figures in the Conservative Party; they were written in Downing Street by a senior adviser of the Prime Minister.

It seems obvious that this was part of the attack programme being prepared by the Labour Party. It was intended to benefit Labour at the next election. It was intended to benefit the Prime Minister, whether or not he had detailed knowledge of any particular line of attack. He knew that Mr McBride was an “attack dog”. One may assume that is why he employed him.

I do not therefore accept Gordon Brown's assurances that he knew nothing of the e-mails. I do accept that he probably did not know the particular details of the attack. But he had worked with Mr McBride, and protected him, since they first met in the Treasury. He also would have known about the “communication group” that met on Wednesday afternoons in Downing Street and was chaired by Ed Balls, the Schools Secretary, with Mr McBride present.

The natural explanation is that this group was intended to do black propaganda against Mr Brown's enemies in the Tory Party, but also in the Labour Party. This was an inner circle of advisers, several of whom had been close to Mr Brown during his feud with Tony Blair. They formed a classic group-within-a-group, run by Mr Balls both for his own political advantage and for that of the Prime Minister.

That is what senior members of the Labour Party say, and I can see no reason to doubt them. Labour politicians who appeared to be a challenge, either to Mr Brown or Mr Balls, have been subject to hostile briefing. They believe that Mr McBride did the briefing, but Mr Balls controlled the operation, and that the Schools Secretary was, and is, the closest confidant of Mr Brown.

A Downing Street source, alienated by what has been going on for a decade or more, made a significant statement to The Sunday Times. He, or she, said that Mr Balls is running a destabilising “shadow operation” inside Downing Street; the purpose is to clear the way for Mr Balls's bid for the leadership if Labour loses the next election; he is said to have protected Mr McBride when other ministers complained; he was in continuous contact with him, sending him up to 20 e-mails a day; he instructed Mr McBride on negative briefings against Labour colleagues.

According to The Sunday Times, at least eight senior Labour figures urged Mr Brown to sack Mr McBride, even before the e-mail scandal. They included Lord Mandelson, Harriet Harman, Alastair Campbell and Labour's election supremo, Douglas Alexander. Ed Balls, or his spokesman, has commented that “these allegations are completely fabricated and malevolent nonsense without any foundation in fact”. Nevertheless, I believe them.

The historic Labour splits have always been based on personalities; they always develop into vicious faction-fighting. They also lead to election defeat. Gaitskell versus Bevan was the root cause of the Labour defeat in 1951, which was followed by 13 years of Conservative rule. The Left-Right dispute of the late 1970s led to the breakaway of the Gang of Four, the creation of the Social Democratic Party, and the 18 years of Conservative rule after 1979. The war between the Blairite and Brownite factions has already lasted for at least 12 years. Now it seems to be moving into an even more vicious second generation. They really hate each other. The feud has become an obsession, much more exciting than the election battle with the Conservatives.

The next general election will presumably be held in May 2010. It may already be lost for Labour. The two latest opinion polls give the Conservatives large leads. The poll in yesterday's Mail on Sunday gives the Conservatives 45 per cent, Labour 26, and the Liberal Democrats 17.

A poll in The Sunday Telegraph is similar; it gives the Conservatives 43 per cent, Labour 26 and the Lib Dems 21. These are landslide figures. Even the slightly narrower margin in the Sunday Telegraph poll would give the Conservatives a majority of 160 over Labour, and an overall majority close to 100.

The Sunday Telegraph poll suggests that it would make little difference to change the Labour leader. Jack Straw would be the most popular replacement; Ed Balls gets only 3 per cent. The Conservatives would be happy to fight an election against Mr Balls, whom they regard as an unattractive bully. They do not expect to have such luck, either in 2010 or later.

The division inside the Labour Party was contained as long as Mr Blair was leader; he was exceptionally attractive to voters. Already, in 1950, Ivor Jennings, the constitutional authority, thought that the office of Prime Minister had acquired a status “almost comparable with that of the President of the United States”. He wrote that “a general election is in reality the election of a Prime Minister.”

Of course, party still remains very important - after 1997, the Conservatives were certainly the unpopular party, whether or not they really were the “nasty party”. The swing of the pendulum is also important. Mr Blair won three elections; he would have found it considerably more difficult to win a fourth. Yet Mr Brown has damaged the reputation of the office of Prime Minister, both by his manipulation of leaks and briefings, and by his dark suspicions of his colleagues. Elections are seldom won by unpopular parties with unpopular leaders in a difficult economic period.

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