Mary Ann Sieghart: Long live the king – if he's harmless

If Charles doesn't wind his neck in, the public won't put up with him. The institution is only there by public consent

Forget Hello! magazine and Hollywood starlets. This is the kind of bling we really love: crimson and gold, prancing horses, diamond tiaras, glass carriages. So much classier than an Oscar ceremony. It stirs feelings of pride and patriotism, not least because royal pageantry is one of the few things that Britain still does best. And it brings a brilliant flash of colour into a Northern European country that can often look grey.

Although in politics I am a moderniser, I am still at heart a monarchist. And there is no reason why the two should be incompatible. You can support the Alternative Vote, gay marriage and reform of the drug laws while still thrilling at the pomp and spectacle of a parade of stagecoaches down the Mall. For, as long as the institution of monarchy does no harm, and quietly modernises as it goes along, we can safely appreciate all the virtues it brings.

An elected president in a grey suit wouldn't have set our hearts aflutter. He wouldn't have brought streets and villages together to enjoy a bunting-bedecked party at which neighbours could make new friends. He wouldn't have caused millions of people around the world to turn on their TVs and marvel at Britain's ability to spin real-life fairy tales. But, you may reply, presidents are at least elected. If they are bad, we can chuck them out. That's true, but the downside of democracy, in this case, is legitimacy. Presidents feel they have earned the right to exert power and influence. The great thing about our monarchy is that it has no power and very little influence. It is our elected politicians who run the country, not our head of state.

There was a time when our kings and queens had real power. They had enormous egos, believed they had the divine right to rule, and were worshipped as semi-deities. As the power of the monarch has dwindled, so by necessity have the egos of the monarchs. And this is a good thing.

The Queen's father, George VI, was a modest man who struggled simply to do his duty, stammer out a speech and raise the spirits of a nation at war. The Queen has continued his tradition. She seems almost devoid of ego, happy to eat her breakfast cereal out of Tupperware and to do her job with the utmost blandness, subsuming her own character in the interests of her calling. There must be days when she groans at the prospect of yet another dull ceremony, but she gets on with it, as impassive as ever, rarely betraying impatience or boredom. She must know how much pleasure her presence brings to hundreds of thousands of people who will never forget the day she visited their school or charity or community centre.

Kate Middleton seems to have the same understated, almost ego-free, approach to the job. Unlike Princess Diana, she is not remotely attention-seeking. Yes, we were transfixed by her dress and her dignity, but not because she was inwardly shrieking, "Look at me!" If she manages to continue the cipher-like tradition that the Queen has so successfully followed, then the British public will be able to project whatever fantasies they like on to her without fear that a real, complicated persona will get in the way.

Let's hope William is the same. So far, he seems to be remarkably grounded and unspoilt – particularly surprising given his mother's neuroses, the rancour between his parents and his traumatic bereavement. He also seems healthily aware of the need to bring the monarchy up to date.

If the throne were to pass directly from the Queen to Prince William, which is what more than half the public wants, its continuity would be assured. One popular monarch would be replaced by another. There is, however, a little obstacle to this smooth transition. It is pretty much unthinkable that the Royal Family would agree to throw over tradition and skip a generation.

Prince Charles has many virtues. He cared about the environment long before it was fashionable. But despite being well-meaning, he can hardly be described as ego-free. On the contrary, he often comes across as spoilt, petulant and demanding.

If that were all, it would be unfortunate but not disastrous. The biggest threat Charles poses to the monarchy is his determination to get involved in politics. It is emphatically not the job of the heir to the throne to be a political campaigner. Yet he has criticised government policies in health, education, foreign policy and human rights. He has even recently published a book, Harmony, in which he sets out a personal political manifesto and describes it as a "call to revolution".

This is dangerous territory, and if Charles isn't careful, he might find himself at the wrong end of a quite different revolution. For the monarch simply must not have strong political opinions. As the great chronicler of the British constitution, Walter Bagehot, wrote: "The Crown ... is commonly hidden like a mystery, and sometimes paraded like a pageant, but in neither case is it contentious. The nation is divided into parties, but the Crown is of no party."

So will King Charles put aside his own views and shrug off his ego when he reaches the throne? Somewhat ominously, he told Vanity Fair last year: "I think people don't quite understand how much it requires to put your head above the parapet ... but I just feel deeply. I always have done. So I can only assume that it's something that's sort of inherent."

If Charles doesn't wind his neck in, though, the public won't put up with him. For however undemocratic the monarchy looks, the institution is only there by public consent. While we want it to continue, it will. If we decide we have had enough, we can vote it out. One bad monarch, who fails to respect the traditional neutrality of the post, could be enough to finish off a thousand years of history. And that would be tragic.

The failure to invite Tony Blair and Gordon Brown to the royal wedding was a bad portent. Rumour has it that Charles and Camilla blackballed them. This would have been a disgracefully small-minded gesture: all families have to put up with the maddening uncle or the nightmare cousin at a wedding. But, worse than that, it looked nakedly partisan. The monarchy can't afford to make such a mistake.

Nearly 60 years on, we still have no idea what the Queen thinks about anything. Her aloofness has served the country well. Yet Prince Charles, if he were a commoner, would probably have launched his own political party by now. As someone who wishes the monarchy well, I devoutly hope he can shrug off his activist mantle and assume the studiedly non-partisan position of his mother. Then we can carry on enjoying – with a touch of playfulness and irony – the colourful spectacle of the Royal Family, without any fear of their becoming embroiled in our democratic politics. Charles needs to re-read Bagehot: "A royal family sweetens politics by the seasonable addition of nice and pretty events." Not, please note, by taking political sides.