LONDON, NOV. 21 -- Embattled Prime Minister Margaret Thatcher today defied intense pressure from members of her divided Conservative Party to surrender her leadership and vowed anew to run in a second ballot against challenger Michael Heseltine despite indications that some of her support may be melting away.

Thatcher, returning to her office at 10 Downing Street after three days at the European security meeting in Paris, spent several hours listening to confidential advice from senior party leaders. She then emerged to tell reporters: "I fight on; I fight to win."

A few minutes later, Thatcher's lieutenants formally submitted her name for a second ballot, nominated, as last time, by senior cabinet secretaries Douglas Hurd and John Major, the two men tapped as most likely to enter the contest if she had chosen to drop out.

Their support for Thatcher effectively rules out a third candidate and sets up another political showdown next Tuesday with Heseltine, the former defense secretary who won enough votes in the party's House of Commons caucus to deny her a victory in Tuesday's dramatic first ballot.

In choosing to run again, Thatcher ignored a warning from the party's chief legislative whip, Timothy Renton, who told her this afternoon that she no longer commanded a majority of the party and could lose the next round to Heseltine, according to parliamentary sources.

Several supporters said they wished she would take the hint from Tuesday's disappointing vote result and resign to allow another candidate to enter the contest and try to stop Heseltine, a maverick Tory lawmaker who has gathered strong momentum over the last week.

"What colleagues have been saying to me is simply this: They would like a wider choice, and they feel that could help clear the air more than just leaving it to the two to slug it out at the OK Corral next Tuesday," said Geoffrey Johnson Smith, vice chairman of the party's 1922 Committee, the main organization representing Tory lawmakers.

Supporters of Thatcher argued that only she has a chance of stopping Heseltine. If she withdrew, they contended, the challenger would quickly grow strong enough to overrun a last-minute candidacy of Hurd or Major. Some even hinted that she might be prepared to resign with dignity next spring and step aside for Hurd or Major, provided she first defeated Heseltine.

The 1922 Committee's 20-member executive split down the middle today in its own private meeting, according to sources. It voted not to make a recommendation to either candidate to withdraw.

"If she sticks to her decision, I think probably she could win," said Johnson Smith. "Whether she could win convincingly, that is the big question mark."

Other supporters said they would have to reconsider how they would vote in a new round. "I voted for the prime minister . . . {but} the position now is that I and many of my colleagues are having to think very seriously about what we're going to do next week," legislator Hugo Summerson told BBC Radio.

Heseltine's campaign aides, elated over his strong showing, said they had gathered commitments from 10 former Thatcher supporters who said they would vote for Heseltine in the next round.

Thatcher outpolled Heseltine by 204 to 152, with 16 abstentions, but fell just short of the 15 percent margin she needed under party rules. In a second ballot, a simple majority will suffice.

Heseltine predicted that a substantial number of lawmakers would defect to his camp. Before the first ballot, he said, his candidacy had suffered from "a very substantial credibility problem . . . but the moment they then see you with 152 and another 16 abstentions, then the whole thing becomes credible."

The result seemed to stun party leaders, most of whom had predicted a Thatcher win. Some were further upset when just seven minutes after the announcement of the vote, Thatcher emerged from a private suite in Paris to tell reporters -- without having first consulted any of those leaders -- that she would run again.

Heseltine, who has criticized Thatcher's unilateral style of government, said she should have consulted cabinet colleagues and party elders before jumping into a second round. "I personally, instinctively, as somebody who is absolutely determined this party should be united and consulted . . . would have preferred that route," he told BBC.

Thatcher, 65, has served as party leader for 15 years and prime minister for 11. After speaking briefly to reporters this afternoon, she rushed off to the House of Commons, where she conducted a spirited review of the Paris summit. Then she made her first appearance in months at the Commons tea room, chatting briskly with members of Parliament, some of whom appeared embarrassed to see her there.

Her office announced that Energy Secretary John Wakeham, a senior cabinet member, would take over running her campaign, which both sides said had been lackluster during the first round. On Thursday, she is to lead her party on the House floor in debating Labor Party leader Neil Kinnock's motion of no-confidence in her government.

At the Commons this afternoon, Paddy Ashdown, leader of the minority Liberal Democrats, said Thatcher's premiership was in "the twilight days" and warned it would not be good for Britain to be represented at crucial conferences on European economic and political union next month "by a prime minister who has lost her authority and a government that has lost its way."

Thatcher responded: "The first 11 1/2 years have not been so bad, and with regard to twilight, please remember there are 24 hours in a day."

Some backers said they gained heart from her performance. But David Howell, one of Heseltine's senior supporters, said it was too late for her to recover. "I don't think you can stop an avalanche halfway," he said.

Thatcher got some moral support from President Bush, who said in Paris he had been impressed with her steady performance at the summit gala after suffering a damaging setback at home. "I'll tell you, to show up there in the wake of a traumatic electoral process of this nature I thought in itself showed her fiber and her steel," Bush told reporters.