It is hard to equate the soft, warm, Irish accent of Brian Mawhinney, as he deals politely but firmly with John Humphreys on the Today programme with the hard man seen by his former civil servants. Like the best of his countrymen, he can turn on the charm, when he likes. But the new Conservative chairman has a killer instinct, which is one of the reasons why John Major appointed him.

This week's success in Walsall - putting the boot into the local council and wrong-footing the Labour leadership - confirmed Dr Mawhinney's private reputation as "Mr Nasty". On the day of his visit, the town's local paper interviewed himunder the headline: "Hard Man in for the Kill with Labour." Such headlines have rarely been accorded to a Tory party chairman since Norman Tebbit was in the job.

The Tory Party chairmanship has generally gone to smooth, unflappable politicians, who were good on the box, particularly after by-election disasters: Peter Brooke, Ken Baker, Chris Patten, Sir Norman Fowler and the disaster-prone Jeremy Hanley, otherwise known as Mr Blobby.

John Major tried Hanley for his skills at chatting up the blue-rinse brigade of Conservative ladies on the "rubber chicken" circuit, but he proved hopeless at the business of beating up the Opposition and the backbenches. Mr Major needed someone to put some steel in the Tory backbone. The attack on Labour in Walsall was just what the doctor ordered.

It came as no surprise to his former Whitehall minions. The politest thing they say about him is that he does not suffer fools gladly. Few tears were shed when he left the Northern Ireland Office, where he was responsible for security. "He is not an easy person to like," recalled one official.

As an Ulster Protestant, with a powerful ambition, he believed he should have been the next Secretary of State for Northern Ireland after Tom King. Although he promoted mixed schools, Dublin regarded him as too Orange. Irish sensitivities probably cost him that Cabinet job.

He was moved sideways instead to the Department of Health to give Virginia Bottomley some heavyweight support. His medical training -he was a senior lecturer at the Royal Free Hospital - meant he knew his subject, better than those around him, and he could not resist letting his officials know it. Civil servants responsible for sloppy thinking were likely to be called in for a public carpeting. "I don't mind a bit of bullying by ministers. Sometimes they have to do it to get the best out of civil servants, but he seemed to enjoy bullying them in front of others," said one.

Fond of catchphrases, he will often say: "Who's in charge?" and "Are you telling me?" delivered with menace and a grin like a shark, which leaves little doubt about the answer. His other preoccupation is: "Is it Paxmanproof?"

He showed no disloyalty, publicly or privately, to Virginia Bottomley, though it must have irked him that she was in the Cabinet and he was not. Loyalty is one of his great strengths, particularly valued by John Major. "He's someone I would want on my side. I wouldn't like him against me," said a ministerial colleague. "He is a thug. He has a black and white view of life, and there's a sneering tone that is never far below the service. He's not what you'd call a nice man, but he does earn a lot of respect."

Dr Mawhinney was one of the close advisers who told Mr Major to face his critics in the leadership election. After helping to run his campaign, he was an obvious choice for the Central Office job - he was keen for a chance in the post that others were equally eager to avoid.

The prime minister and party chairman both entered parliament in 1979, and have neighbouring constituencies. They have consolidated a friendship, perhaps because they recognise similar qualities in each other. Both have stable family relationships and a largely concealed ruthless streak, which has got them where they are today. Above all, Mr Major knows he can depend on Dr Mawhinney to support the line from Downing Street and not to foment dissent from Smith Square.

As a committed Christian and a former member of the general synod - he has a well-developed sense of right and wrong, which he brings to his view of socialism. He did not much hold with Mrs Bottomley's liberal leanings.

The now-defunct Health Education Authority never recovered from his banning of a safe sex guide for young people, on the grounds that it was "smutty". "His idea was `don't do it'," said one official. He also had no time for liberal attitudes to drug addiction, shocking workers at a progressive clinic in Liverpool by telling them to get the users permanently off drugs, and get the Church involved.

The Church was often involved with health promotion. "We used to always try and get a bishop, and if we couldn't get a bishop, we'd get Cliff Richard."

Dr Mawhinney, even when relaxing, rarely indulges in anything stronger than a glass of beer. After he left the Department of Health, his successor, Gerry Malone, found a stock of Lucozade Lite in his ministerial cupboard, which he quickly replaced with Oddbins' Muscadet.

Dr Mawhinney felt overdue for promotion when he was brought into the Cabinet as Transport Secretary in 1994, at the age of 54.

He quickly set about defusing some of the problems he had inherited from John MacGregor, changing the rhetoric in favour of a greener transport policy. There was little he could do about British Rail privatisation but his big policy pronouncement was to ditch the widening scheme for the M25 which greatly relieved the Tory population of the stockbroker belt - at last someone was listening to them.

One of his first tasks at Central Office was to rearrange this year's forthcoming Tory Party annual conference to keep the limelight on messengers bringing "good news" - Kenneth Clarke, Michael Heseltine, himself, and the PM.

Last year, as Transport Secretary, he used a catchphrase for his conference platform speech which he borrowed from the Irish comedian, Jimmy Cricket. "Listen... there's more..."

He said it with a beckoning finger, and a smile, which could put a chill down the spines of the most hardened socialist dinosaurs. Next week, he will be saying it to Labour in the North-east, Tony Blair's backyard. The party workers at Smith Square are delighted. "He's on a roll," said one.