MARGARET Thatcher remains Britain's prime minister, at least for the present, but her inadequate victory in her Conservative Party's caucus leaves her severely weakened. While she won more votes than her challenger, she won too few to secure a decisive reelection to the party's leadership on the first ballot. That means there will be another ballot next week. From the party's point of view, it is the messiest and least welcome of outcomes, as it leaves a wounded prime minister in office while the struggle over the succession widens.

This upheaval comes at an awkward moment, for the British government is going to have to make major decisions over the coming month. Some of them will affect Persian Gulf strategy, in which Mrs. Thatcher has been the most adamant of President Bush's European allies and the readiest to talk about resorting to war. There are also crucial decisions immediately ahead on the economic and political union of the European Community.

But over the past year there have been growing indications that neither her country nor most of her own party were with her in her stubborn hostility to European union. That's the issue that led to this challenge to her in the caucus, and it's crucial to Britain's future. Whether the timing is convenient or not, it's better to have it out before the country has to choose whether to stay in Europe wholeheartedly or hang behind. That's not a decision to be made by a government that does not demonstrably have a substantial consensus behind it.

The United States depends on Mrs. Thatcher's sturdy help in the Persian Gulf and would clearly prefer not to see a change of command now. But a change would be unlikely to affect British policy toward Iraq. Mrs. Thatcher's adversary in this week's vote, Michael Heseltine, has said that he intends no changes there. Another candidate, so far unannounced, is Mrs. Thatcher's present foreign secretary, Douglas Hurd, who has been an active participant in the deployment of British forces in the Gulf.

Mrs. Thatcher, who took office two years before Ronald Reagan did, has been in power longer -- far longer -- than the head of any other major government in the world today, and longer than any British prime minister since the Earl of Liverpool, who was a contemporary of Napoleon. She has changed her country and done great things for her party. But now that party senses she cannot win another election for it, and suddenly she is on the defensive.