'Kings were put to death long before 21 January 1793," wrote Albert Camus, referring to Louis XVI's execution. "But regicides of earlier times and their followers were interested in attacking the person, not the principle, of the king. They wanted another king, and that was all. It never occurred to them that the throne could remain empty for ever."

As New Labour prepares for its own regicide, it is vital to make a distinction between people and principle. The desire that many in the party have to march Tony Blair to the guillotine is entirely understandable. For the past nine years he has shown his contempt for both the principles on which the party was built and the institutions that built it. It makes sense that some of those who hold those traditions dear would seek to respond in kind. What is not so clear is the haste for the next coronation. The issue has been framed as if what Labour needs is another king, rather than a thoroughgoing discussion about whether the regal tendencies that have emerged in the past decade serve either party or country well.

So attention has been focused on when Gordon Brown will succeed Blair - rather than whether it would make any difference to anyone beyond the two men in question and the parliamentary posses they have gathered around them. This focus bypasses the very question the Labour party needs to answer now more than ever: namely, what does it stand for apart from office?

For if the public is to engage with this succession, the contenders must give them something to engage with. The one requirement of personality politics is that either the personalities or the politics have to be interesting. The rivalry, for example, between Dominique de Villepin, the French premier, and Nicolas Sarkozy, the interior minister, for the right's presidential nomination represents a genuine difference in French conservatives' response to globalisation. Similarly the rift between Ariel Sharon and Binyamin Netanyahu in Likud had real implications for Israel's future.

Conversely, there are some personalities who are so compelling that they maintain our interest regardless of their politics. Watching Berlusconi hyperventilate in the run-up to next week's Italian elections - describing himself as "the best political leader in Europe", comparing himself to Jesus Christ and promising to abstain from sex until polling day - has been enthralling. As is Hugo Chávez, Venezuela's president, who persuaded Julio Iglesias and Jiang Zemin, the then Chinese president, to join him in a rendition of O Sole Mio, and who informed his wife on national television the day before Valentine's Day: "Marisabel, tomorrow I'm giving you yours."

These are characters to whom few can remain indifferent. Indeed their impetuous natures inform, to a degree, their political agendas and fortunes. Yet in the Blair-Brown battle we are treated to neither significant political differences nor particularly engaging personalities.

The question of what Brown would do differently deserves substantial answers; without them the country's political future is reduced to the career aspirations of two middle-aged men trapped in a dispute over who promised what job to whom over dinner more than a decade ago. The issues of thwarted ambition and fear of post-retirement melancholia that have shaped the tiffs on both sides would be better resolved on a therapist's couch than in cabinet meetings. Last week's uncorroborated accusation, that Brown cancelled the pensioners' council-tax discount to sabotage the local elections and hasten Blair's demise, was a new low. Headlines like "Your mum's old Labour, Blair tells Brown" cannot now be far off. All this seems to have as much to do with politics as Biggie and Tupac's rap wars had to do with music. In Ice T's words: "It's a personal beef that got out of whack."

So long as the Tories were imploding this mattered little. But now that the opposition has found a palatable candidate and a workable formula, promoting both social and economic liberalism, the purpose of New Labour will be subjected to greater scrutiny. And rightly so.

So far New Labour has proved itself capable of winning elections, a crucial talent lacking in previous decades. But so have Hamas, Sharon, Berlusconi and George Bush. Winning elections is important. But in itself it neither legitimises a programme nor justifies a particular set of priorities. Labour's ability to accede to power is no longer in doubt. But its ability to transform that power into progressive change is no longer even discussed.

There have of course been notable achievements. Investing more in health and education, civil unions for same-sex couples, the minimum wage, devolution and reducing child poverty are all solid liberal advances that may prove as difficult to reverse as Thatcher's anti-union legislation. But more striking is what has remained the same or got worse. A recent survey by the Institute for Fiscal Studies revealed that economic inequality in Britain has remained virtually unchanged under Blair. Our racial discourse is in the gutter. Our foreign policy is a reactionary, murderous mess. Given that Brown was the co-architect of the New Labour project, he must take some responsibility for this.

Admittedly, as the heir apparent, there is brinkmanship involved. Unless you're Peter Mandelson, you only get to resign once, so it is strategically important to keep your powder dry until the moment is right. Keep it too dry, however, and you might shoot yourself in the foot, like Clare Short and Frank Dobson, who found their conscience once they had lost the power to do anything about it.

Brown's instincts, say his supporters, are more attuned to the party's; his stewardship of the economy owes more to social democracy than the market. If he had been running the show, he would have done things differently. Maybe. But if he has been a closet radical all along, now would be the time to come out.

For whether it's vouchers for asylum seekers, tuition fees or identity cards, we have yet to see a matter of principle that could not be subordinated to his own career ambitions. If you're not going to resign over a policy as egregious as the invasion of Iraq (even when doing so would have brought down Blair and likely prevented British involvement), what is the right time? In short, if there is no substantial political rift between Brown and Blair, why bother changing faces? One MP shockingly compared the battle between the two to Stalingrad. But Stalingrad had consequences.

Labour does not need a new monarch but a new mission. Not a return to old Labour, whatever that was, but a renewal of the values of internationalism, equality and wealth redistribution applied to the modern challenges of globalisation, universal human rights and diversity.

"The party is a moral crusade or it is nothing," said Harold Wilson. Blair has shown us the crusades. Now it is time for some morality.