Instead, he elected to do something rather rare. He surrendered a strong hand in case he damaged the political project to which he has devoted his adult life: the election of a Labour government. He decided that the dangers of a personal battle between himself and his friend were too great, and so he walked away from a contest which just might have taken him to Downing Street.
The decision took a lot of agonising, long conversations with friends, including Mr Blair, and much brooding. Mr Brown felt that, had the vacancy occurred earlier or later, he would have been the obvious successor as Labour leader. Instead, he has been toiling to create an economic policy that was sensible, saleable and consistent. This meant an end to easy promises, which bored the media and depressed the faithful - a job as thankless as it may one day prove invaluable. Politics is timing. Politics is unfair.
But the shadow Chancellor emerges from his choice a bigger man. No one will be able to label him a careerist. His 'new economics', based on radical supply-side reforms, not neo-Keynesianism, is endorsed by Mr Blair. With Mr Brown alongside him, Mr Blair is not only likely to enjoy a runaway victory; he will be a more solidly buttressed leader, and a more plausible prime ministerial candidate. If he wins, Mr Brown is guaranteed the run of No 11.
This renunciation does not mean there that will be no Labour contest. At least one of the several formidable people on the left is likely to run against Mr Blair, and rightly so. But the prospect of the modernising wing of the party being shattered in a struggle between its two main figures has gone. It is far less likely that any coming contest will damage the party that John Smith left behind. These are facts that will be of significance right across the political spectrum.
Mr Brown has had a tough few years, jabbed at regularly as a compromiser who has lost his youthful radical fire. His Welsh speech - 'the flame still burns, the work continues' - was fiery enough. But what about the compromises? Mr Brown once published a biography of the Red Clydesider Jimmy Maxton. In it he quoted Nye Bevan castigating Scottish Labour rebels: 'Yes, you will be pure all right. But remember, at the price of impotency. You will not influence the course of British politics by as much as a hair's breadth.'
Despite his image, Mr Brown is a romantic like Maxton. Yet when it came to the moment of decision, it was Bevan's voice that rang the louder.