Alan Howarth has made a bold, brave decision and all praise to him. But he has a bruising time ahead. There will be exhaustion and disappointment, and both the exhilaration and pain of politics. Those of us who formed the SDP 14 years ago at least had the comfort of numbers. ButHowarth will be on his own, and lonely.

There may be 30 or 40 other Tory MPs who broadly share his views, but it will be a surprise if any join him. A few will remain his friends and invite him to join them for a drink. But most will say he should have stayed to fight, even when they have done precious little fighting themselves. When he visits the House of Commons - which he must do as soon as it returns - they will slip away as they see him approaching them along the library corridor, making an unexpected visit to the Gents to avoid him. In the Smoking Room they will turn their backs.

The constituency will be difficult. He will be called "traitor" in the street. He will be accused of using the Conservative Party as a stepping- stone to a status he does not really deserve. There will be obscene, anonymous postcards written in bright colours with a felt pen. Most difficult of all, there will be the sad, tearful faces of those who respected - perhaps even loved - him who now feel abandoned.

The leadership of the party will be too shrewd to call him "traitor". They will prefer to diminish him: "We all know Alan ... quite a good junior minister ... but never really significant." He will be the sheep that lost its way.

Howarth will feel most uncomfortable in his decision to remain as MP for Stratford-on-Avon. He is right in calling constitutional convention to his defence and quoting precedent. When the SDP was launched, this was the position we adopted. But even supporters found it difficult to explain.

He knows, of course, that if he fought a by-election and won, he might not survive a general election. Alternatively, if he simply resigns and does not stand, the seat would probably go to the Liberal Democrats, who were runners-up last time. Tony Blair's heavy men will have explained how unacceptable that would be.

Howarth must cast his mind ahead. Perhaps, early in the next century when the Tory party has moved back to the centre, there will be those who will say, "You helped to do it, Alan. Our party has now become the party you wanted it to be all those years ago." After all, it is the SDP that helped Blair create the Labour Party that Howarth believes is now fit for him to join.

Lord Rodgers of Quarry Bank was a Labour MP and founder member of the Social Democratic Party.