PARIS -- Margaret Thatcher treated this Monday in Paris like another working day, even though it risked being her last full day fully in power. If Thatcherism is a spent force, as is now being said in London, the Iron Lady has been working hard to show that the news has not reached her.

She took breakfast with George Bush to see if the American presidential spine needed more stiffening on the Persian Gulf. Seemingly satisfied with the resolve of Potus, as the president of the United States is known in government cables, she then called a press conference to refute the conventional wisdom that the Conservative Party will clip her wings -- or even remove them altogether -- in a vote on the party leadership today. Rubbish, she calmly tells skeptical British reporters.

Thatcher must win the party vote to remain prime minister. She must win it convincingly if she is to remain the domineering force in British and international politics she has become over the past decade.

Between Potus bracing and press bashing, Thatcher puts in the required appearance at the 34-nation pan-European summit she and Bush are here to attend. She proudly points up her differences with the European federalists who would place their trust in European conflict-resolving mechanisms rather than in NATO. Real Britons do not floss or think European, her brisk manner conveys to the audience back home.

But the message is not as winning as it once was. "Europe" is draining her once unchallenged power like a pair of headlights left on negligently pulling down even the most powerful battery. On Europe, Maggie stands alone and vulnerable in her own councils of power.

Rubbish, her spokesman, Bernard Ingham, repeated Sunday night when confronted with this line of argument. Look at all the leaders here in Paris asking to see her. Yes, replied a British reporter sweetly, they want to say goodbye.

The souring of a nation's romance with a larger than life political leader resembles the breakup of a once happy marriage in more ways than one. Outsiders usually can fathom little about the true causes or the exact moment when the bust-up will come. The French never understood why Richard Nixon had to go in 1974. Americans puzzled over the French voters' rejection of and ingratitude toward Charles de Gaulle in 1969. And we who focus on Thatcher's towering achievements in foreign policy find it difficult to grasp fully why she was suddenly seen as vulnerable within her own Tory constituency.

Unlike the Republicans of George Bush and the Socialists of Francois Mitterrand -- and most other ruling parties in industrial democracies these days -- the Conservatives of Margaret Thatcher proved that it does matter which party rules. They have overhauled Britain and reshaped it in their image, for better and for worse.

Michael Heseltine says that the policies he opposes cannot be changed without a change of the leadership of the party, and he is right. Margaret Thatcher has shown that in her country it does matter who runs the government.

It is a characteristic she has shared with the other larger than life politician here at the pan-European summit, Soviet President Mikhail Gorbachev. They got on so well in part because they recognized in each other an unshakable determination to make a difference, in their own terms, in their countries. Now they both are on the list of endangered political species.

British voters have not suddenly become enamored of continental federalism. But on each trip to Europe they see and understand that despite the financial and economic gains Thatcherism has brought, British society now lags behind the continental Europeans in many important respects. Britons read in the newspaper last week that while 80 percent of French boys and 90 percent of French girls aged 17 are still in school, the comparable figures in Britain are 30 percent for the boys and 40 percent for girls.

After a decade in power, Thatcher can no longer deflect responsibility for the obvious decay of the British education system and the collapsing public infrastructure onto the situation she inherited from the Labor party.

From a narrow self-interested standpoint, Americans should wish she could. As she has shown again in the Persian Gulf crisis, Thatcher is America's most consistent, reliable ally abroad. She has shared the best of America's impulses abroad and acted on them, to the point of causing many Britons to resent the care and attention she gives to the special relationship with Washington. Those resentments will be a problem for America in a post-Thatcher Britain.

Her crucial role in the Gulf crisis is a strong and urgent argument for serious Conservatives to keep Margaret Thatcher in office now. She is part of that vital message being sent to Baghdad. She can also argue to the party that the effects of her decision to take Britain into the European currency exchange system will not be clear until next spring. Neither will the work of the monetary and political union conferences of the European Community.

"It is not time yet to write my memoirs," she said confidently here yesterday. The Conservatives should listen carefully to the argument for not putting the Iron Lady on the junk heap at this moment.