Conservatives who have pondered it are horrified, and understandably so. In yesterday's Daily Mail, an anonymous cabinet minister is quoted as warning: ''People need to realise if that happened there would not be another Conservative government for 25 years.''
Quite. People do need to realise that.
The thought is provoked by Paddy Ashdown's speech last night, which was as strong an invitation to the Labour leader to sit down and talk about a political reformation as I have heard. On education, welfare reform, Europe, the environment and economics he laid out a series of principles with which Blair would agree. But he made it crystal clear that voting reform is part of the price for co-operation.
Assuming, as I do, that Blair is serious about his centre-left revival, and truly believes that the large majority of voters want a pro-market but welfarist Britain, tilted towards Europe, then he ought to be at least half-attracted to voting reform. If his politics are really the consensual centre ground, then a proportional system would entrench them, not undermine them. Blairism couldn't lose.
Ashdown is offering him more than the short-term, jobs-for-the-lads deal of the Seventies. The Liberal Democrats' leader is suggesting long-term and secure parliamentary backing for key policies, without necessarily requiring seats in the Cabinet or a formal coalition. His speech implies that he would also support a Labour government from the outside, as a loyal opposition.
This is generous enough, I suspect, to cause Ashdown problems with some of his MPs. It would be particularly valuable to Labour if the party won only a small majority. Blair desperately needs that extra time and security if his ideas on stakeholding, greater investment, political reform and so on are to be implemented and dug in enough to show some return.
As Ashdown put it: ''What we have to build in Britain must be robust enough to survive for at least two parliaments and strong enough to carry a programme of fundamental reform against the power of the entrenched vested interests that will oppose it.'' Such a deal wouldn't be a distraction or a dilution for Labour's social policies. On most of them, the Lib Dems are already more radical. In some circumstances, it could be a precondition for Labour's other agenda.
Blair must also have pondered the effect of voting reform on the British Union. The divergence in political mood of Scotland and England has been strongly accentuated not only by the Home Rule issue but also by the first- past-the-post system, which makes the English south look more Conservative than it is and Scotland more socialist.
As Robin Cook has pointed out, more people voted Labour in Kent at the last election than in Glasgow; and more voted Labour in the English south (excluding London) than in Scotland and Wales put together. This truth is hidden by the voting system; PR would make different parts of the country look more politically alike, and would hence have a unifying effect. For a Labour Party worried about the effect of Scottish Home Rule on its longer- term Scottish representation at Westminster, this must be attractive.
Last, but certainly not least, there is the likely effect on the Conservatives of raising the issue of voting reform. A change of this magnitude would have unpredictable effects on all parties. But a proportional system would cause most problems for the Tory coalition. Electorally, both Tory One Nationers and the Thatcherite nationalists would have less reason to continue tolerating one another. Under PR, both factions could hope to win seats in the Commons by themselves. As the Mail's unnamed minister realised, the Tory party would find this a powerful disintegrative force.
There, then, is the self-interested Blairite case for embracing voting reform; it would buy him the time he needs to prove himself a serious prime minister, while damaging his enemies more than his own party. It is the sort of bold, imaginative stroke that one can imagine Disraeli delighting in.
Some will complain that it is also unprincipled, and that Blair sees himself more as a Gladstone than a dirty Dizzy. But almost everything that the average politician says about the voting system is unprincipled. When Tories praise the current system for offering stability, what they mean is: ''us in power for keeps''. When Liberal Democrats rail against the frustration of the people's will, what they mean is: ''us lot kept out''.
This is fair enough. A voting system is neither a thing of beauty nor a joy for ever. It is a mechanism, a tool whose shape skews the politics of the country that employs it. That's all. If Blair needs some voting reform principles to disguise a brutal demarche, he can pick them up quickly enough.
So why would he hesitate; and what does he really think? Whenever I have talked to him he has seemed coolly sceptical about voting reform, though careful not to commit himself against it. His advisers are split, I guess 50:50.
He has every incentive not to show his hand yet. He knows that if he announced his conversion to reform, he would infuriate some key colleagues with whom his relationship is already problematic. It would be futile to split Labour before an election in pursuit of a policy designed to split the Conservatives afterwards. We may recall that Neil Kinnock also seemed hostile because he, too, was obsessed by the possibility of a pre- election split, yet we now know that he was a private convert to voting reform.
Blair may not be another Kinnock on the issue. But I suspect that he is more open-minded than he lets on, or than some of his advisers would like. He has held strongly to John Smith's promise of a referendum - strongly enough to twist some union arms before the last party conference in order to prevent a vote against it.
More recently, he has sanctioned private frontbench talks with the Liberal Democrats on political reform, and, in an interview with my colleague Donald Macintyre, has said that he would take a view in any referendum on voting reform. Is it thinkable that, in the turbulent midst of his reforming administration, Blair would find himself campaigning alongside Michael Portillo and the High Tories, in the ''no change'' camp?
I used to think so, but I am changing my mind. The implications could not be bigger. If a referendum mandated reform, there would follow a Commons battle of heroic scale, in which every Tory MP returned in 1997 would join with Labour last-ditchers against the change.
I think the anonymous minister, warning his colleagues of a possible convulsion ahead, was spot on. If Blair is offered a chance to make history, he may hum, haw and hesitate. But eventually, he'll strike.