LONDON -- For Conservative Party members of Parliament, it has suddenly and agonizingly come down to this: Whether to jettison Margaret Thatcher, the world-renowned leader who has presided over 11 unbroken and triumphant years of power or to risk going down to electoral defeat alongside her.

None among them wanted this dilemma. Even those most firmly set against the continued party leadership of Thatcher had hoped she would read the handwriting and go gently. But few are surprised that she has chosen instead to stand and fight for party leadership in a secret ballot Tuesday of the 372 Conservative members of the House of Commons.

Most analysts believe she is likely to win, but many also believe the race is certain to further wound her even if it vanquishes her main party rival, former defense secretary Michael Heseltine.

"I find it very painful," said Graham Riddick, a young, first-term Tory from a marginal seat who says he will vote for Thatcher but concedes the potential damage to her, to himself and to their party. "It is unnecessary and divisive and very unfortunate indeed, and I wish it wasn't happening."

Heseltine is seeking to engineer a very British coup, similar to the one Thatcher herself brought off in 1975 when she unseated Edward Heath, then the party leader. Thatcher's fate will be decided by mostly middle-aged, mostly male lawmakers meeting in secret in a small room. To win, 214 must vote for her.

They looked like conspirators this past week as they gathered in oak-paneled corridors and huddled in tearooms in twos and threes, talking in hushed tones and falling silent whenever a stranger neared. Few felt comfortable discussing publicly the pressures and fears working on them. "We're all cutting and running when asked these questions and ducking around the next pillar," said Emma Nicholson, one of the few to confess publicly that she is undecided.

Throughout it all, there is an air of pained disbelief, a sense that what is being asked of them is too difficult, too quick, too momentous.

"What's happening is so extraordinary that I can't see how it's ever going to be possible to put the lid back on again," said Austin Mitchell, a lawmaker from the opposition Labor Party who watched with awe as his Tory counterparts engaged in the same kind of internecine warfare for which his party was once famous. "I can't see how she can recover from this damage, let alone how all the hatreds and antagonisms can be shoved back into the bottle and sold to the customer."

For the Conservatives, the nightmare started as an argument between Thatcher and her most senior cabinet colleague, Geoffrey Howe, over Britain's future role in Europe, a question that deeply divides Conservative lawmakers.

But after Heseltine declared his candidacy Wednesday for party leadership -- and thus for prime minister -- he quickly switched the terms of the contest to an issue with more direct political clout: Thatcher's new system of local taxation, the highly unpopular so-called poll tax. He pledged "an immediate and fundamental review" of the tax, if elected.

Still, as far as Heseltine's backers are concerned, the real issue is Thatcher herself, her abrasive personal style, her perceived inability to adapt to changing times, to admit mistakes and bend.

"It's not an issues question, it's a leadership question," said William Powell, a Heseltine loyalist who says his man's support cuts across ideological and generational lines. "She's been a great prime minister, and we all wish to see her go out with dignity, but the truth is, she's worn out. She's reached the point where everyone else is wrong and she's right. None of us want it, but there comes a point when you simply have to say 'enough.' "

Local polls and private tallies suggest that Thatcher is well ahead, but because everyone's vote is secret, no one's vote is certain. Last year, when Thatcher faced a nominal challenge from Sir Anthony Meyer, 24 lawmakers spoiled their ballots by voting for both Thatcher and Meyer. That way they could honestly tell colleagues that they had supported the prime minister while still expressing some form of protest against her.

On a regular parliamentary vote, in which the results are public, the party's hyperactive and all-knowing whips can be highly effective in holding maverick lawmakers in line. But the carrot-and-stick approach is far less effective when the balloting is secret. The Daily Telegraph quoted an anonymous Tory as dismissing the whips in this vote: "They come up to you in the tearoom and grab you by the arm saying, 'Can I count on your support?' Like all good Tory MPs, I say yes to everyone."

Another Tory centrist says privately that he plans to abstain but is telling the Heseltine camp he will vote for them in the hope that if Heseltine wins, he will get a ministerial post.

Thatcher starts out theoretically with a base of at least 150 votes: Close to 100 from those on her payroll -- cabinet secretaries, government ministers, senior staff members and whips, plus perhaps 60 from the hard-core right wing of the party. But even these groups are not a sure thing. Some of her younger ministers are openly hostile, while a handful of the rightists say they have reluctantly concluded she cannot win and will only succeed in electing a Labor government if she does not quit now.

The 21-member cabinet is expected to support her, but even there the commitment appears less solid than public statements indicate. Douglas Hurd, the politically moderate, smoothly articulate foreign secretary who seems to be almost everyone's compromise candidate, has ruled himself out of the contest unless Thatcher steps aside -- leaving the door open if she does. Equally revealing was his quasi-endorsement of Howe's devastatingly critical resignation speech in the House of Commons last Tuesday. "I agreed with a lot of what Sir Geoffrey Howe said," said Hurd. "I just didn't agree with his conclusion."

Faced with both the uncertainty and the danger of further backlash, Thatcher's camp has become restrained, her own voice unusually calm and noncombative. On Tuesday, while the balloting takes place, she will be in Paris attending the summit meeting of the Conference on Security and Cooperation in Europe, basking in the role of world leader and implicitly defying the men of Parliament to dare unseat her.

Her lieutenants both at Downing Street and in the Tory central office a few blocks away are under strict orders to keep their criticisms of Heseltine civil and impersonal. As for Howe's wounding resignation speech, Thatcher's aides will say only that she was "surprised and saddened." They give no hint of the anger and the sense of betrayal other insiders say she feels for Howe -- or her sense of icy disdain for the challenger.

Heseltine, a millionaire publisher who has stalked Thatcher ever since he stormed out of her cabinet in 1986 in a dispute with her over the takeover of a failing helicopter firm, starts with a base that is much smaller than Thatcher's but perhaps more secure. He can count on most of the 60 or so legislators who voted for Meyer or abstained in last year's race, plus another 25 or so hard-core supporters. If he can push that total above 100, analysts say, that will be enough for him to declare himself at least a respectable loser.

The most unsettled group in this race are those who favor neither Thatcher nor Heseltine but would prefer a third choice, such as Hurd. Heseltine has invited their support, noting that if he manages to deny Thatcher an outright victory in the first round of balloting but does not win it himself, Hurd or other candidates could enter for the second round a week later. "If people believe that Douglas would be a better leader . . . then of course they have to vote for me on the first round, because that is the most likely way of assuring Douglas has a chance," Heseltine told BBC radio. Many of them may wind up abstaining, hoping that their votes combined with outright votes for Heseltine will shake Thatcher enough to cause her to resign.

Thatcher loyalists such as Graham Riddick find it hard to believe that the contest has come this far and that their leader is so close to defeat. He believes that when lawmakers leave the hothouse atmosphere of Parliament this weekend and return to their home districts to consult local Tories, they will hear solid support for their leader.

But some districts seem as divided as their elected representatives. Kenneth Hargreaves, a centrist Tory who says he is undecided, listened silently Thursday night as a BBC-TV newscaster talked with a half-dozen local Conservative leaders back home in his Hyndburn constituency. Two Thatcherites and a Heseltine supporter were interviewed. Then a defeated candidate in last spring's local council elections described the hostile reception she got while canvassing modest row houses whose owners had paid $160 in property taxes last year and then suddenly found themselves facing $900 poll-tax bills.

Then the program turned to Hargreaves, who looked as if he were about to cry. The poll tax, he admitted, was "a political disaster." He promised to go home this weekend and listen to his constituents.

But if the dissension displayed on the air is any indication, Hargreaves will not find the answer in Hyndburn. He and his parliamentary colleagues will have to search for it within themselves.