Funny business, politics. In 1988 Paddy Ashdown became the first leader of the Social and Liberal Democrats after promising they could replace Labour. Eleven years on, the plain old Liberal Democrats have enjoyed triumphs unparalleled since Lloyd George's days, but Labour has had the last laugh.

Westminster was baffled last night about exactly why Ashdown, who always said he saw the job as a long haul, has quit now. Thanks to Labour, his party has achieved many of its aims: devolution for Scotland and Wales, House of Lords reform, many non-Westminster elections to be held using proportional representation. Even PR for general elections, the Lib Dems' Holy Grail, is a possibility.

Perhaps the sheer scale of Tony Blair's parliamentary majority helps explain why Ashdown has suddenly, unexpectedly, now chosen to stand down. Any hope the Lib Dem leader had of his party holding the balance of power, and using its leverage to force through PR and a radical centre-left realignment of British politics, disappeared on May 1, 1997.

Alternatively, it could be that Ashdown feels his dream of a ''new politics'', involving a permanently ''progressive'' Lab-Lib government, will happen anyway, even without him, and ''the project'' is safe in the Prime Minister's hands. The two men are close personal friends and political soulmates, and both loathe what they recently labelled the ''destructive tribalism'' of yah-boo British party politics.

Or it may simply be that after 11 years Ashdown has tired of the fray, especially at Westminster, which he regards as a pompous and unreal place. Even Commandos retire from the battlefield. In recent months he has appeared increasingly tired, uneasy, and irritable. He was keenly aware of the rising tide in both parties against the ever-closer Lab-Lib links he and Blair pushed hard to foster.

When he got to his feet at Prime Minister's Questions last week, scores of Labour MPs booed and baited him. Dennis Skinner raised a laugh when he shouted in Blair's direction: ''sack 'im!''. Ashdown tried to deflect these insults with sarcasm, but his anger showed through.

At his party's annual conference in Brighton last September the same frustration was apparent in unguarded moments. While he was listing and lauding the practical benefits of close co-operation with Labour, many Lib Dems - far too many for his liking - voiced opposition arguably born of the ''tribal'' mindset he wants to banish.

Whether the Lab-Lib love-in survives Ashdown's departure depends partly on who takes over. Both the leading contenders, Charles Kennedy and Simon Hughes, favour looser links. The former wants the Lib Dems to replace the Tories as Britain's main Opposition party; the latter wants to target disaffected traditional Labour voters, greens and the marginalised.

History will judge Ashdown as a superb leader of his party. When he took over, David Owen had just quit politics, the newborn SLDs were getting poor ratings and, embarrassingly, fared worse than the Greens in the European elections. Irrelevance beckoned. Now they have 46 MPs, their highest number since 1929, and, despite their support for many Government policies, still often provide a more coherent and credible Opposition than the Tories.

Under Ashdown, they scored spectacular by-election successes - including Kincardine and Deeside in 1991 - and gradually improved their representation in Parliament and in town halls across Britain. In July 1996, Ashdown even overtook Blair as the party leader with the highest public satisfaction rating. His 1992 confession of a five-month-long affair with former secretary Patricia Howard led the Sun to famously dub him ''Paddy Pantsdown'', but full disclosure ensured the scandal was short-lived.

In policy terms, Ashdown's Lib Dems affected to maintain ''equidistance'' between Labour and the Tories. But that evaporated as co-operation with Labour, already evident through the Scottish Constitutional Convention, was extended UK-wide under Blair's leadership after 1994 through the ground-breaking Robin Cook-Bob Maclennan constitutional talks.

With Blair in Downing Street, the Lib Dems practised ''constructive opposition'' and were given a rare taste of real political power through their joint membership with Labour of the unprecedented Joint Cabinet Committee. However, when Blair and Ashdown issued a surprise joint statement last November recommending widening the JCC's work far beyond constitutional issues, many Lib Dem MPs' alarm-bells rang.

Paddy was being too much the military man: leading from the front, going too far too fast, giving orders, and not explaining the mission. Hardly anyone had been consulted; hardly anyone liked it. This Indian-born, Ulster-reared ex-diplomat had got his tactics wrong.

In recent months, Lib Dem MPs have begun saying quietly that the single-mindedness with which Ashdown transformed the party's fortunes had become an arrogance. There has been growing resentment at his tendency to rely on the advice of a small coterie of confidants such as Lord Holme, Paul Tyler and Nick Harvey.

Ashdown told Tyler, his Chief Whip, of his plans before Christmas. MPs of all parties were amazed that, in the Westminster gossip factory, nobody had an inkling before yesterday afternoon's shock announcement. It was news management to perfection. It seems all those visits to 10 Downing Street and cosy chats with the PM paid off after all.