If new proposals are enacted, every dog in the country will be fitted with a microchip bearing a barcode that reveals the owner's name, address and number, as well as the pet's name, breed, age and health. Far-fetched as the plan seems, it is the latest instalment in the unlikely story of the dog in British political life, metaphorically or otherwise.

When the battle was raging over the power of the House of Lords almost a hundred years ago, David Lloyd George, the Chancellor of the Exchequer in the Liberal government, called the Lords "Mr Balfour's poodle". In truth that wasn't quite right: their lordships' house was no mere pet of A.J. Balfour, the Tory leader and former prime minister. Although it sometimes suited him to let the Lords nip the Asquith government, he found the Upper House far from tame or easily leashed.

Then the best part of a century later, the same dog appeared as a regular phrase. Towards the end of George W. Bush's presidency, when his disastrous Iraq war had made him an outcast shunned by almost all foreign leaders, the witty Maureen Dowd of the New York Times imagined "W" sitting morosely in the White House with no friends or companions left except for Barney and Tony, "his Scotch terrier and his English poodle".

That was how Tony Blair seemed to many on both sides of the Atlantic. Still, it leaves few apt canine metaphors to describe the humiliating treatment Gordon Brown has just received from Barack Obama in New York, veritably a case for the RSPCA.

parliament in Jeremy Thorpe's constituency as the candidate of the Doglovers' Party. This was an elaborate tease on Waugh's part against the leader of the Liberal party, who had been accused of a plot to silence someone who claimed to be his former boyfriend, who was threatening to ruin his career. Whether or not there was serious attempt to kill the young man, there was no doubt that his dog Rinka had been shot dead, and Waugh's straight-faced candidacy was a not very gentle reminder of that sad event.

When the last Tory government passed the Dangerous Dogs Act, it led to endless difficulties for police and courts. That should have served as a warning against hasty legislation in response to general panic, but the warning has not been heeded, as the lamentable law aimed at child abuse now shows. Parliament might bear it in mind before they try to shove a microchip into every dog in the country.

No one has lately called Lord Mandelson Mr Brown's poodle. If anything their relationship is the other way round, with the business secretary today – such are the revenges brought by time's whirligig – in a position, most commentators agree, to make the prime minister sit up and beg, or even have him sent on a last journey to the vet.

On Sunday, Mandelson admitted that Labour were "underdogs". He went on to say that they could still bark and bite, as it were, or at any rate that Labour might yet win the election. But then he has also dropped broad hints that he would work with a Tory government, and David Cameron will be aware that this is no ordinary lapdog growling.