A couple of weeks ago, I stumbled upon the best political fact I've come across in a long, long time. And because we've grown quite fond of each other, you and I, I intend to share it with you.

Nelson Mandela attended focus groups. He sat behind a one-way mirror in the Durban townships, while his American pollster quizzed voters about what they thought of him. When he ran to be President of South Africa, he wanted to know what his compatriots felt about the ANC leadership. And he was chastened by what he heard, learning of disappointment and alienation. It profoundly affected his conduct of the subsequent campaign.

I do so wish I had known this earlier. I've had so many arguments with people who believe that focus groups are the enemy of principle. I could have ended them all with this simple fact about the man who told the Rivonia courtroom that he lived for his ideals but, if necessary, was willing to die for them.

This was the best thing I learnt from reading Stanley Greenberg's newly published (and compelling) memoir Dispatches from the War Room, but it was hardly the only thing. In fact, by the time I put the book down I felt I understood how Gordon Brown intends to fight the next general election. And I felt ready to make a guess about the contents of today's Budget.

Stan Greenberg is best known for his pioneering work as Bill Clinton's pollster. After Ronald Reagan's second victory in 1984, Greenberg pitched his tent in Macomb County, Michigan, a Catholic suburban working-class area. He set himself the task of discovering why these core Democrat voters had deserted the party. And he discovered their anger at the way the party seemed to ignore their economic woes and was preoccupied with elitist cultural liberalism. They also saw it as pandering to ethnic minorities.

Greenberg began to advocate a populist Democratic strategy centred on the economic concerns of low-paid white workers. And this brought him and Bill Clinton together. He was one of the small team that helped turn the Governor of Arkansas into the President of the United States. Based on Greenberg's polling, Clinton's campaign manager, James Carville, scribbled a note on the whiteboard to remind staff what the election would be fought on. “The economy, stupid,” the notice famously read.

Less well known is that these words were not the first on Carville's list. Above “The economy, stupid” came this: “Change versus more of the same.” And this, as much as the economic populism, was classic Greenberg. Using intensive polling, Greenberg's approach to campaigns is to try to find a dividing line between his candidate and the opponent that rings true and leaves his man on the winning side. His job is to listen carefully to the team's ideas, to come up with a position for his guy and a way of characterising the other guy and then to test the hell out of it.

The reason I am telling you all this is that in 1995 Stan Greenberg took on a new client. He became pollster to the Labour Party.

Immediately he set to work on the usual Greenberg campaign - listen, identify the dividing line, test. Partly driven by Labour's advisers and partly by his own instinctive economic populism, Greenberg began to home in on this phrase: “Labour works for all the people. The Conservatives work for the privileged few.” He was comfortable with it. In tests it seemed to work brilliantly. There was just one problem. Tony Blair wouldn't have it.

For about 50 pages of his memoirs the pollster lays out in agonising detail his attempt, through both the 1997 and 2001 campaigns, to persuade Blair to accept this economic narrative. But Blair could not be persuaded. “I just don't believe the problem with Britain is the few at the top,” he said. When Greenberg later asked why he wasn't simply fired, Blair replied: “You were giving me the right advice for an election. It's just that I was choosing not to take it.”

There was someone else who was much keener on Greenberg's “many versus the few” campaign: Gordon Brown. Believing passionately in dividing lines and the political power of economic populism, Brown hired a fellow believer, the American speechwriter Bob Shrum, as his own advocate of such campaigning.

Dispatches from the War Room suggests Brown was so keen on the economic populist idea that he “did an end run” straight to Greenberg's Washington office to press the case. And the pollster also reveals that Brown's famous attack in 2000 on Oxford University's supposed class bias came the day after a very unhappy meeting at Chequers. Yet again Blair had rebuffed the idea of a “many versus the few campaign”. Brown tried to launch such a campaign on his own. He did not succeed, but never quite gave up, later drawing Greenberg into his circle.

Reading these passages, I think I understand Gordon Brown's next move. He will, because he always does, fight the election around a dividing line. He cannot (successfully) use the “Tory cuts versus Labour investment” on which, after all the internal wrangling, the party's exhausted strategists fell back in 2001 and 2005. The public finances won't let him. Nor can he really fight it as the “more of the same” portion of a “change versus more of the same” election. His “no time for a novice” gibe suggests he was attracted. But once the threat of a future economic crisis has passed, “more of the same” is simply the wrong side of the election argument, as any pollster will quickly tell him.

So he is left with his first love - a “many versus the few” campaign. It must be very tempting. Bankers bonuses, a party questioning capitalism, a chance to invigorate his base, the absence of an alternative, David Cameron's background: all this lures Brown down the path Blair wouldn't take.

My guess is that today's Budget will attempt to sharpen this divide. The Chancellor will increase taxes on high earners and dare the Tories to reverse them. Last time he tried this George Osborne refused to play ball, announcing that reversal of tax rises on high earners was not a priority.

If he understands Brown's election campaign, Osborne should be sure to do this again.

daniel.finkelstein@thetimes.co.uk

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