For nearly a dozen years, Britain's government has been remarkably stable. Prime Minister Margaret Thatcher has held office all that time, but her rule suddenly looks very shaky as she is being challenged for the Conservative Party leadership by her former defense minister.

In this week's party vote, Thatcher did not lose, but she did not win, either. Party balloting gave her a 204-152 verdict over Michael Heseltine. That's a 9.7 percent margin of victory. Unfortunately for the prime minister, party rules say the margin must be 15 percent.As a result, a second party vote will take place Nov. 27. But the uncertain result the first time around shows that Thatcher is vulnerable - something no one would have believed just a year ago.

If Thatcher loses the leadership of her own party, she would resign as prime minister.

In that eventuality, the United States would be deprived of one of its strongest supporters. Thatcher has been a solid backer of U.S. foreign policy decisions and America's most outspoken and ready-to-fight ally in the Persian Gulf crisis. It would be unfortunate to lose that relationship.

Thatcher's political support at home has eroded over the past year or two as British citizens - who enjoyed many boom years under Thatcher - have become increasingly restive over a poor economy, high unemployment, and unpopular taxes. Within the Conservative Party, there has been dissatisfaction with Thatcher's reluctance to join more enthusiastically in the Common Market move toward European union.

The prime minister may yet triumph. But the timing of the party revolt against her leadership comes at an unfortunate moment as far as the United States is concerned.

Even if Thatcher is defeated in the party vote and resigns as prime minister, Britain's policy in the Persian Gulf probably would not change significantly for now, but U.S. diplomats are fearful it would send the wrong signal to Iraq's Saddam Hussein.

Saddam might take the downfall of one of his most outspoken Western foes as a sign of disagreement or weakness in the Western camp. That, coupled with congressional criticism of President Bush, might encourage the Iraqi dictator to continue his refusal to withdraw from Kuwait in the belief that the people of Britain and the United States are willing to let him get away with it, despite what their leaders say.

The United States has no say in what Britain's Conservative Party decides to do about its leadership, but American officials would feel a sense of relief and satisfaction if Thatcher were to come through Nov. 27 as a clear winner.