The Eurocity Tower in Lille is trembling, and nobody knows why. Last week it had to be evacuated. It isn't a seismic problem but comes from within. The authorities say: "Something inside there is emitting a frequency which the building itself is picking up and amplifying."

Many readers this weekend will have the same feeling about the United Kingdom. For the last few weeks it has been shaking. And the cause of the shakes - the Diana demonstrations in England, the Home Rule votes in Scotland and Wales - is not any outside force but a mysterious pulsing of energy deep inside the old tower. The people, who have shown that they are a real creature and no abstraction, want change.

And yet they have chosen a very odd moment indeed to start pulsing and shaking. For, if I'm not wrong, this country's politics are entering a period, probably a very long period, of almost geological stability. By the time it ends, columnists will be asking desperate questions about the absence of democratic challenge and the atrophy of choice.

Never mind the 18 Tory years from 1979 to 1997. They may come to seem a roller-coaster of alternating triumph and disaster compared to the solid, benevolent permanence of the Blair epoch. "Events", those unpredictable nuisances that used to upset Harold Macmillan, can always make nonsense of this prophecy. But the evidence on show at the moment points only one way. Nobody seems quite to realise the immense durability of the landscape that the voters created on May Day 1997.

We are peering into a centre-left eternity. The great May majority will erode a bit. But precedents suggest that New Labour should carry through into a second government, though with a smaller majority, at the elections of 2002 or earlier. Tony Blair's personal popularity and authority in the country appear to be still growing, five months into the new administration, rather than slackening off. "Events" must surely dent him as time passes, but Britain's honeymoon with this man is showing no signs of the customary disillusion. Could this last into a copper or even silver wedding?

More significant still are the shadowy outlines of a centre-left "settlement", broader than a power mono- poly by the Labour Party. The joint cabinet committee in which Liberal-Democrat and Labour politicians consult is only a first token. But Mr Blair did not need this innovation, still shocking to many of the rank and file on either side, to get his way in the next five years. He set it up with a view to the distant future. Assured of Lib-Dem loyalty, he could seriously hope to rule for a generation.

This is the embryo of coalition politics. If the Government goes ahead in a few years' time with a referendum on a proportional voting system and wins it, then a Lib-Lab coalition will almost certainly be born as a formal institution. That sort of government would inaugurate a pattern familiar in Germany, for example.

There, for almost all the Federal Republic's 48-year life, two big parties - the Christian Democrats (CDU) and Social Democrats (SPD) - have alternated in power through coalitions with the smaller Free Democrats. And that is the sort of political framework we seem to be heading for. The electoral system to be used by the Scottish parliament - two votes, one for a single- member constituency, one for a regional party list - is a rough imitation of Germany's. If the Scots feel comfortable with it, and if the British vote for PR, it's the most likely form to be adopted for Westminster elections. And its outcome will, pretty certainly, be a Lib-Lab coalition which could prolong centre-left government into the 2020s.

British debates about coalition government follow much the same tramlines in the Oxford Union or the Dog & Duck. There is the moan about small parties forcing big parties to betray their manifestos. There is the groan about coalitions making a mockery of the will of the voters. Then there is the odd person out, usually a woman, who gets up and says that adversarial politics are ridiculous and it's high time politicians paid regard to what unites them rather than what divides them. End of debate! I don't deny that there is something to all these points. But if coalitions are coming, then the British ought to be learning the crude rules of how to get along with them.

Voting habits will change, for a start. The two-vote system we are likely to get tempts people to vote for a constituency candidate from party A and for the list of party B. In other words people rapidly learn to vote for a coalition rather than a party. Many Germans, for example, habitually vote SPD or CDU for their local MP, but Free Democrat for the list. They want a government led by Social Democrats but with Free Democrat ministers to prevent any socialist extremism (hardly a danger with the SPD). Or they want Christian Democrat conservatives in charge of the economy but Free Democrats to ensure that civil liberties are not reduced.

We will see if the Scots use their two votes like this. Pundits who assume that they won't split their vote are in for a nasty surprise. Those who vote Labour with their first vote will often throw their second vote to the Scottish National Party or the Lib Dems. The parties likely to harvest both votes from their supporters are the SNP and the angrily isolated Scottish Tories.

Paddy Ashdown and the Liberal Democrats will have to learn the basic rule of being the junior party in a coalition: that change is effected by treachery. The decisive event in German politics is when the Free Democrats defect from one big partner and join the other.

This can make the third party look shabby. After some years in partnership, it can begin to seem that the party has no principles, only desperation to stay in power. And when most of its voters come to see the party as a mere coalition ingredient, the necessary ketchup on the chips, the party begins to lose "profile" with its core supporters and its hold on their votes slips away. The best defence against this sort of decline is personality. A big, dramatic figure holding a senior post in the coalition - Paddy Ashdown as Foreign Secretary - can at least slow this process down.

But an opposition has to learn new rules, too. The first is to hug the centre. There will be no more landslides like 1979 or 1997. Victory depends on small percentage changes in the vote, and on tireless work to conciliate and eventually detach the junior coalition partner. There is no sign that the Tories have grasped this. Neither have they realised that regional devolution will be crucial for an opposition. Here, in a Wessex or Anglian parliament, is the chance for a Tory administration to show the nation what a good government it could form at Westminster. Recruiting the aggrieved periphery against the centre will be the key opposition strategy.

But none of this is inevitable. If the "Great Peace" of endless centre- leftism is to swaddle Britain like a duvet, Tony Blair must sort out one or two things. First, the monarchy has to be given new batteries. If it conks out halfway down the road to constitutional reform, the whole programme will be blocked. Second, Scotland. If devolution is not managed with the tact and craft it needs, the Scots could slide into independence, and Blair's grip on Westminster might not survive the crisis.

And, third, the Liberal Democrats. You can never tell with Liberals. You can plan a useful future for them, but they can suddenly go off-message. They can decide to die on their feet rather than live on their coalitionary knees.

This would rudely interrupt the centre-left eternity. But it might, one day next century, be just what democracy needs.