By Nick Assinder
Political correspondent, BBC News website
It may run against the popular image of the two men's relationship, but Tony Blair is doing Gordon Brown a big favour by staging the vote on Trident at this time.
Few issues can split the Labour Party so bitterly and comprehensively as policy over Britain's nuclear weapons system.
Mr Blair says Trident decision must be now
So, when Mr Brown carefully revealed his support for its replacement last year, he fanned the flames and infuriated some of his left-wing supporters - notably Clare Short, who declared she could no longer support him as leader.
Now, two junior members of the government have quit over the issue with rumblings there may be more to come.
It has become one of the battlegrounds in the leadership campaigns with would-be leaders Michael Meacher and John McDonnell, and deputy candidate John Cruddas, opposing the move.
There has been a flurry of Commons motions against the government policy as it stands and anti-nuclear groups are staging demonstrations around the country.
It all looks set to culminate in a sizeable Labour rebellion and the government once again only getting its way thanks to Conservative support, something no leader likes but which Mr Blair seems to be getting used to.
The arguments for and against have been well-rehearsed and there is the usual whips operation under way in the Commons to limit the size of the revolt.
It is already likely to be split between those opposed outright to the proposal, and those backing a move to simply delay the decision.
There have even been suggestions that very junior government members - Parliamentary Private Secretaries - will not be sacked if they abstain rather than vote against the government.
Party bosses are still trying to sway some rebels, and there have been warnings about a return to the bad old days of the 1980s when similar rows contributed to Labour's years in opposition.
Protests have greeted government policy
That is not a package the new leader and prime minister - likely to be Mr Brown - would want to have to deal with in his first months in the job, when he is attempting to impress his party and voters with a radical and positive new programme.
So it suits his purposes that the prime minister is arguing that the decision must be taken now, because it will take 17 years to design, build and deploy a new Trident submarine system.
Mr Blair seems eager to sort it out on his watch, so it becomes part of his legacy, so both men are winners.
One of the consequences of pressing the issue now - just as pressing ahead with plans on nuclear power - will be to clear the decks of the issue before the chancellor takes over.
That is not to say Mr Brown will not suffer collateral damage from the row.
His support for the policy did not surprise most MPs but it still angered many, mainly natural allies, who believe it is the wrong policy for our age, or that it is a decision which needs far greater debate.
They believe Mr Brown has acted too quickly and too eagerly. Mr Brown will not want resentment over his decision to linger when he replaces Mr Blair - assuming he does.
The other worry for the Brown camp is that, in the final days of the Blair regime, backbenchers get a taste for rebellion.
That is partly because of the contentious issues being pushed through in the prime minister's final days but it is also a result of Mr Blair's declining authority.
The fear for the next leader will be that, once acquired, the habit of rebellion is hard to kick.