The insurance industry, stung by the D.C. City Council's preliminary vote to end a mandatory no-fault car insurance program in the District, has launched an eleventh-hour campaign to try to persuade the council to reverse its decision when it meets tomorrow.

The targets of these efforts are council members Nadine P. Winter (D-Ward 6), a one-time supporter of no-fault who unexpectedly changed her position two weeks ago, and Carol Schwartz (R-At Large), a relative newcomer to the council who opposes no-fault because she claims it has resulted in higher insurance premiums.

The insurance industry last week mailed letters to all 50,000 registered voters in Ward 6 and to every registered Republican in the city, most of whom live in Ward 3, Schwartz's Northwest Washington home base. The letters assert that insurance rates in the city would increase by 11 to 60 percent if the council's preliminary action stands. Within two days of the mailing, Schwartz received about 900 calls and Winter received about 150.

The council is scheduled to take a final vote on the insurance amendments tomorrow, and some council members predicted that the 8-to-5 vote that gave initial approval to the amendments will not yield under the latest pressure from the insurance industry.

Some insurance industry representatives said they are convinced that Winter's vote reflected a successful lobbying campaign by lawyers who represent accident victims in lawsuits.

"The lawyers continued to go to the mayor and the council behind the scenes," said Grover E. Czech, regional vice president for the American Insurance Association. "They have been able through their quiet lobbying to change the vote of Winter and to convince Schwartz."

Winter insists that she made up her mind because of pressure from her constituents, who she said want the right to sue to collect damages, and not because of lobbying by the trial lawyers, whose business would increase if the no-fault provisions were eliminated.

"No-fault really has not produced what it was supposed to produce," said Winter. "I don't think there is any groundswell in my ward for it. My constituents look forward to the day they can sue. It is like hitting the numbers. When you take the right away from people to sue, they feel hurt."

Winter said she could not estimate how many people in her ward are dissatisfied with the no-fault law and that she was trying to get statistics from insurance companies as she prepared a written statement for the record on her vote.

"What she said during the last council meeting is nonsense," said August P. Alegi, a vice president for Government Employees Insurance Co. (Geico), the biggest auto insurer in Washington. "Winter told me she was going to change. She told me she didn't want to hear about logic. She kept repeating that her constituents wanted to sue."

Under the current system, no-fault coverage allows accident victims to be compensated for their medical expenses by their own insurance companies, regardless of who is at fault in an accident. The law prevents a person from suing for pain and suffering unless the victim has incurred at least $5,000 in medical bills or suffered disfigurement, disability or death. The proposed changes would make no-fault coverage optional and eliminate the $5,000 threshold. Insurance would remain mandatory for all motorists.

Winter's vote in favor of the changes has puzzled some because she led the fight in 1982 to adopt the no-fault law. On Nov. 5 she was crucial in blocking an effort to table amendments that would make no-fault coverage optional. Council member William R. Spaulding (D-Ward 5), a former no-fault supporter, also backed the changes. Yet some insurance industry representatives said recently they are convinced that Spaulding was merely following Winter's lead. He has not been targeted by the insurance industry.

"It was a major surprise to me," said council member H.R. Crawford (D-Ward 7), who tried to table the proposed changes. "We really will not have no-fault now."

Schwartz said her decision was based on her research. "People are paying for double medical coverage through health and auto insurance policies, and I have a philosophical problem with people saying no one is at fault," she said. "I feel very comfortable with the vote I made."

The amendments approved by the council on first reading were proposed by Mayor Marion Barry, who said the changes are needed because a U.S. District Court judge's decision struck down the $5,000 threshold in a case that is now on appeal. Barry noted that no major reductions in insurance premiums have resulted from no-fault insurance.

"I think people thought the no-fault issue had been settled, and I was shocked," said council member Betty Ann Kane (D-At Large), a supporter of no-fault insurance. "In two years, I've gotten six letters on the subject and they were the result of prodding by lawyers. I think the council's vote is a disgrace and a real disservice to consumers."

Some council members and insurance industry officials who declined to be identified said they thought that Winter's vote may be tied to her plans to run for reelection next year -- an assertion that Winter disputed last week.

In the past, trial lawyers have contributed handsomely to candidates who supported their views. After the no-fault law was adopted in 1982, lawyers contributed more than $20,000 to the campaign of Sterling Tucker who ran against then-council Chairman Arrington Dixon, a no-fault supporter.

At that time, a spokesman for the trial lawyers said that a number of attorneys wanted to "show their protest to Dixon."

This is the second time in recent months that Winter has played a key role in a controversial consumer issue. In April, Winter voted with six other council members to make major changes in the rent control law.

Edward L. Norwind, who lobbied for the no-fault amendments and is the past president of the Association of Plaintiffs Trial Attorneys, said his group has taken no position on whether to contribute to Winter's 1986 campaign. He noted that the insurance industry has been the group to spend large sums of money.

Geico advanced the $24,242 to mail letters to Schwartz's and Winter's constituents, according to Czech of the insurance association. He said it had spent a similar amount in a separate mailing to 42, policyholders. "They [insurance companies] have put a scandalous amount of money into this issue," said Norwind. "I don't think we made the difference. The difference is on the merits."

Although he said he could not supply any statistics, Norwind said lawyers have seen an increase in complaints from consumers about delayed payments from insurance companies.

"We believe there should be no locks on court house doors for people who want to make claims," said Norwind. "Our objection is to the really oppressive power that insurance companies have over consumers under the no-fault law." On the other hand, Geico's Alegi insisted that trial lawyers see only the people who have problems, while hundreds of people receive payments promptly. Alegi said the insurance amendments would produce direct benefits for lawyers.

"They ought to call it the trial lawyers relief act," Alegi said. "It is only going to make them wealthier than they are."

Several council members say that the insurance industry's mass mailing is an attempt to frighten consumers with predictions of higher insurance rates.

The letters sent to constituents of Schwartz and Winter were signed by the Alliance of American Insurers, the American Insurance Association, the National Association of Independent Insurers, the D.C. Federation of Civic Associations, the D.C. Legislative Committee of the American Association of Retired Persons and the National Capital Area Transportation Federation.

"Mrs. Schwartz is new to city government and believes she is doing something good for her constituents even though she's received numerous letters from individuals and civic and community organizations urging her to vote to keep the current no-fault system," said the letter sent to Republicans.

Schwartz called the letter condescending and said that most of the initial callers were angry because they had had trouble reaching her. The letter had listed an incorrect telephone number of her office. She said she convinced some people that the insurance industry is wrong.

"I want people to look at the source of the letter," said Schwartz. "Since when did the insurance industry have the best interest of consumers at heart? I think the bill we passed is a good one. If they can show me one policy where the premium has gone down since no-fault became law, then I will reconsider my vote. I made that challenge to the insurance industry about a month ago."