Insurance companies, expressing concern that taxing employe fringe benefits would harm workers, employers and themselves, are gearing up a direct-mail lobbying effort similar in scale to the massive campaign mounted by the banking industry two years ago to repeal a law requiring withholding taxes on interest and dividends.

They fired the first shots in the battle yesterday, holding a press conference at which officials of organized labor, a consumer group, a business group and the life and health insurance industries said taxing fringes "is bad, unwanted social and economic policy."

Two insurance trade associations also released a commissioned poll showing that 79 percent of Americans surveyed opposed taxation of fringe benefits even if the trade-off were lower tax rates, a central premise of the tax-simplification plan proposed by the Treasury.

The life insurance industry "sees it as an extraordinarily serious threat to our business, and we're going to fight as hard as we know how to try to see that the Treasury proposal does not get enacted," said Edward E. Phillips, chairman of the American Council of Life Insurance and chairman of New England Mutual Life Insurance.

"We have a number of agents around the country who are reasonably exercised about these proposals, and they're very likely to be heard from. Many companies are going to be writing to their policyholders . . . asking them, if they're interested, to write to Washington. . . . We intend to make it a major political campaign."

For the campaign, the ACLI has hired Intromet, a division of the Christmas Club of Easton, Pa., which did much of the mass mailings in late 1982 and early 1983 for the banking industry on the withholding law. Those efforts produced 22 million cards and letters from constituents protesting the legislation, which had been enacted just one year before. Congress repealed withholding in mid-1983.

Intromet Vice President and General Manager Michael McNab said the campaign will be "somewhat similar" to the one used in the withholding fight. The company will send information kits and postcards to major insurance companies and 130,000 independent underwriters. The postcards, which can come preprinted with the name of senators and congressmen, will be passed on to customers in premium notices, checks and other communications.

Although workers don't always realize it, premiums paid by employers on behalf of their workers for health insurance and life insurance are income to the employe that is specifically exempt from taxation. The Treasury proposal calls for taxing health insurance premiums to the extent that they exceed $70 per month for an individual and $175 per month for a family, as well as taxing life insurance premiums paid by employers. The proposal also would end the exemption for such other employer-provided fringe benefits as day care, tuition reimbursement and commuting costs.

The Treasury says its health insurance provision would affect only 30 percent of American workers; industry officials said yesterday it would be more like 55 percent to 60 percent. They also contended low-income workers would be harder hit than those in higher brackets. Robert Georgine, president of the building and construction trades department of the AFL-CIO, said taxing benefits "would seriously threaten the existence of employe benefit programs."

Of the two principal congressional simplification plans, one sponsored by Rep. Richard Gephardt (D-Mo.) and Sen. Bill Bradley (D-N.J.) would also curtail the tax-free status of insurance premiums, although in a different way. Another plan authored by Rep. Jack Kemp (R-N.Y.) and Sen. Bob Kasten (R-Wis.) would leave most insurance-related fringes tax-exempt.

Supporters of tax simplification contend that it is unfair for the government to subsidize workers who are lucky enough to work for companies providing benefits while other workers and the self-employed must pay for health and life insurance with after-tax dollars.

Burns W. Roper, whose polling firm surveyed a sample of the general public and, separately, a group of corporate chief executives for the insurance industry, said the results showed both groups "are in total agreement that the cost of employe benefits should not be taxable to the employe." They opposed taxing fringe benefits -- when the question was asked without other details of the Treasury plan being mentioned -- by margins of 77 percent for the public and 80 percent for the business executives.

Hard-core opposition to taxation of fringes was considerably smaller, however, when respondents were asked how they felt if taxation of fringes were used to reduce the deficit or lower tax rates. In that context, 33 percent of the general public and 50 percent of the executives remained opposed. A separate question asking about taxing health insurance costs as a way of lowering tax rates drew 59 percent opposition from the executives and 79 percent opposition from the public.

ACLI President Richard S. Schweiker, former secretary of Health and Human Services, said the insurance industry would be hurt if the Treasury simplification plan became law but that losses of revenue and profits had not yet been estimated. The $100 billion in increased revenue that Treasury predicts from its plan gives an idea of the magnitude of the effect, he said.