London - Whether Prime Minister Margaret Thatcher wins or loses Tuesday's Conservative Party leadership election, her days of real political power are almost certainly waning.
The challenge by former Defense Secretary Michael Heseltine will inevitably leave her weaker, and there is a growing sense that, one way or the other, she has had her day.
If she wins, she can be expected to lead the party into the next general election and then, if victorious, step aside. If she loses on Tuesday, her 11 years in Downing Street will come to a bitter and inglorious end.
She will be the first serving prime minister to be thrown out of office by her own party this century. But that will certainly not be the main mark she leaves on history.
Mrs. Thatcher can now lay unquestionable claim to being one of this century's outstanding British leaders. Her singular achievement has been to transform the country from the depths of post-colonial malaise into a modern-minded society. Things will never be the same again.
She has reasserted Britain's role in the world. She is the West's elder stateswoman, now more respected abroad perhaps than at home.
She is greeted as the most trusted ally in Washington, receives a heroine's welcome in Eastern Europe and was the first to recognize Mikhail S. Gorbachev as a man the West could do business with. Even in the European Community, whose unity she is trying to restrict, she is listened to, if not lauded.
At home, she has restricted the scope of government. She has diluted the old cradle-to-the-grave statism with a considerable splash of self-reliance. She has created a generation of home- and stock-owners, the bedrock of her "popular capitalism." She has turned an outdated industrial economy into a high-tech service-oriented one.
Thatcherism has not only changed the face of the nation, but it has also colored the complexion of British politics. The opposition Labor Party today bears little resemblance to the classically socialist organization it was when she trounced it in 1979. Its metamorphosis is largely attributable to the impact of Thatcherism.
The power of unions has been curbed, both inside and outside the Labor Party. Labor has retreated from its high-spending programs and dropped its commitment to renationalization of all privatized industries. It has recognized the folly of unilateral nuclear disarmament and been converted to pro-European policies.
It has moved markedly to the right to be in a position to challenge Mrs. Thatcher. The polls suggest its strategy has worked. It has maintained a double-digit lead over the Conservative Party for the last year and is poised for combat in the next general election, which must be called within 19 months.
Mrs. Thatcher's current crisis has three major causes: her strident style of leadership; her anti-European posture; and her introduction of the unpopular poll, or head, tax.
It was divisions within her own party and the country on all these that caused the crisis. The crisis did not cause the divisions.
In issuing his challenge, Mr. Heseltine has zeroed in on these three weaknesses. He advocates Cabinet government, greater European union but not federalism and a fundamental review of the poll tax.
His challenge has thrown into question the prime minister's immediate fate, but not her assured position in Britain's history books.