Had he decided that afternoon to give his leader an ultimatum - either Harriet goes or I do - the uneasy relationship between Mr Blair and his deputy would have collapsed. But that is not all: Mr Blair's relationship with his party would have collapsed, too.
Bur Mr Prescott did not issue his ultimatum. Yesterday he helped to save both Ms Harman's and Mr Blair's bacon. He and his lieutenants let everyone know it was time for the row to stop. But given his obvious unhappiness, why did he act to stem the rebellion? And, more importantly, can Mr Blair rely upon his volatile deputy in the bruising times ahead: is there no limit to his loyalty?
In some ways it is extraordinary that their partnership has lasted at all. They are very different. Prescott is exuberant and ostentatious: he likes his Jaguar and his taste for Caribbean holidays was recently exposed when he got caught on one hit by a hurricane. Blair is more uptight, ascetic, avowedly of the professional classes and less open about enjoying himself. More important, the two men have different relationships with the party. In 1966, when former Cunard steward and trade union firebrand John Prescott was standing for Labour in Tory Southport, the 12-year- old Tony Blair was soliciting votes as the Conservative candidate in his private school's mock election. Mr Prescott is attuned to the older melodies of the Labour Party. His boss is the opposite, accused by left- wing critics like MEP Ken Coates of not having "the faintest idea of how socialists think, or understanding of the mentality of the party that he leads".
Mr Blair appeals to the burgeoning middle classes because he comes from outside the culture of Old Labour. He travels unencumbered by sentimental attachments to the ancient verities of "The Movement". But the journey that is emotionally and intellectually easy for him is deeply troubling for many in his party. It was the distress of this transition that was apparent on Mr Prescott's face two days ago.
Yet the Harman affair was only the latest (if the most serious) of a series of clashes since their election in the summer of 1994.
One of the few perks of a deputy's role is to be confided in, consulted about the key decisions. But on some key occasions in the past 18 months Mr Prescott has found himself well outside it. There was the strategy summit meeting held by Mr Blair and key advisers last spring in the New Forest, to which Mr Prescott was not invited. Nor was the member for Hull best pleased by the leaking of an important memo from PR man, Philip Gould, which warned that Labour was not fit to govern. Mr Prescott had never seen it. Incredibly, this mistake was repeated over the Harman business. The deputy leader first learnt about it from this newspaper. "It was our cock-up," a Blair man told me, "we each thought that the other had told him."
The more important differences have been over policy. Mr Prescott was initially opposed to changing Clause IV, arguing in private that it was a diversion. He is known to have disagreed with Gordon Brown's emphasis on low inflation combined with no tax increases. Last year's conference decision to further cut the union block vote left him fuming. Yet Mr Prescott has a seemingly limitless capacity for loyalty. That stems from a deeply felt and, some might say, old-fashioned sense of duty, that he must do his best for his party even if he disagrees with the policy.
He is prepared to put aside his private doubts and knuckle down once the party position is agreed. So each time he has swallowed his disappointment and got on with the business of selling the new line to the party and the country. On Clause IV, once Mr Blair made it clear that he was intent on change, Mr Prescott threw himself into helping his leader write the new clause, contributing some important sections himself. "He has the argument in private. But when the decision has been taken, John fights for it", says one of his supporters. Mr Prescott is not all selflessness, though. Like most Labour MPs, he is desperate to win the next election and understands that Mr Blair is fundamental to victory.
He yearns for a Heseltine-type Deputy Prime Minister's role, chairing committees on the economic regeneration of the country. So far, Mr Blair has not promised his deputy anything. "He's keeping him keen," one Labour insider believes. "Once it's all in the bag, John may not be so pliable."
Others also believe that part of Mr Prescott likes ripping up the past and starting again. They point to his championing (when shadow Transport Secretary) of private finance to supplement public investment. "John is convinced the Tories stole the Private Financing Initiative from him," says a Brown aide.
Mr Prescott knows Mr Blair has a lot invested in him. A precedent for how vital Prescott could be comes from Margaret Thatcher's days. In 1981, beset by recession, she relied upon deputy Prime Minister William Whitelaw to make Thatcherism palatable to a largely hostile party. He did sterling service to a revolution he did not believe in. Prescott plays that role for Blair.
Does Labour's deputy leader have a bottom line? Could Tony Blair go a modernisation too far? What about electoral reform, pacts with the Liberals, changes to the structure of welfare provision - could Labour's Conscience live with those? History suggests he could. This week he wobbled but eventually came round, reminding his colleagues to fight the real enemy at the forthcoming Hemsworth and Staffordshire by- elections. Provided he is kept informed, allowed to argue his case and Labour is ahead in the polls, Mr Prescott will stay onside.
One of Prescott's main jobs is to rally the party behind the leadership, working the tea-rooms at the House of Commons, keeping in touch with backbenchers. He is vital to hold the party together. Opposition to Blair within the Parliamentary Labour Party is weak and divided. But for the first time this week, we had a glimpse of a possible anti-metropolitan, anti-moderniser alliance within the party. This is a guide to malcontents that Prescott is tasked to keep on board.
Research: Tiffanie Darke and Ben Summers
Tony Benn, Ken Livingstone, Brian Sedgemore, Jeremy Corbyn, Dennis Skinner, Diane Abbott
The usual suspects: long-standing left-wing members of the Campaign Group.
Pro-Clause IV; increased taxation and public expenditure; universal benefits
TROUBLE-MAKING RATING: Will oppose Blair on most issues but unlikely to vote with Tories. Invariably impotent, but could be influential if a Blair government has a narrow majority.
Peter Hain, Angela Eagle, Richard Burden
Mainly younger MPs who believe "modernisation" is code for "right-wing".
Mixed bag. Some libertarian socialists sceptical of Blair's conservative policies on family. Critical of centralisation of power in leader's office. Believe Labour will only carry conviction by being radical.
Can cause short-term damage by attacking leadership. Younger members likely to be attracted by prospect of ministerial jobs.
Gerald Kaufman, Roy Hattersley, Gerry Steinburg, Gwyneth Dunwoody, John Spellar
Long-standing MPs drawn into politics just after the Second World War, wedded to traditional policies on welfare state and economy.
Often anti-European; egalitarian on education; belief in Keynesian interventionism to reflate economy; still hold to goal of full employment.
Feel disgruntled with young modernisers, but unlikely to oppose Blair publicly.
Dennis Canavan, Don Dixon, Peter Snape
MPs who distrust Blair because they strongly identify with their constituencies.
Proudly anti-trendy. Fear over-intellectualisation of the party. Oppose politically correct all-women short-lists.
Disquiet with Blair leadership more about culture than policy - little for opposition to form around.
SINGLE ISSUE IDEALISTS
Frank Field (welfare)
Clive Soley (housing)
Calum Macdonald (Lib-Lab pacts) Tony Wright (proportional representation)
Tam Dalyell (anti-devolution)
Often maverick MPs with policy expertise combined with moral interest in a single issue.
Vary according to issue.
Numerically insignificant but can embarrass