Sen. Bob Packwood (R-Ore.) conceded yesterday that Congress might "buckle under" to the insurance lobby on the issue of sex discrimination the way it did to the banking lobby on withholding of interest and dividends. But he warned that, sooner or later, Congress "will rise up in wrath against this kind of tactics."

At the third and final hearing on the issue by the Senate Commerce, Science and Transportation Committee, its chairman held aloft a copy of an advertisement that has begun to appear in newspapers around the country.

The ad states that legislation to eliminate rate differences based on gender would cost women $360 million more a year for life insurance and $700 million more for auto insurance. Signed by an industry group called the Committee for Fair Insurance Rates, it urges people to write their congressional representatives.

Packwood called the "alleged facts wrong" and said that while such mail attempts to give Congress the impression of qualified public opinion, "all we are going to get is a quantity of letters that in no way represent a sampling of a valid cross-section of public opinion."

After the hearing, Packwood said he was still hopeful a compromise could be reached along the lines that were discussed before the American Council of Life Insurance reversed its position last week. Robert N. Houser, chairman of Bankers Life, repeated that ACLI originally had agreed to unisex rates only because "there was a gun to our heads."

That gun was retroactivity, or the equalization of premiums and benefits in all policies. Now that total retroactivity for all existing contracts appears to be on its way out of the bill, "we can go back to our principles" of opposition to unisex rates, Houser added.

The hoped-for compromise--which Packwood said the committee is still negotiating with several major insurance companies--would require insurers to equalize benefits only for completed contracts, or paid up policies held by retired persons. At the same time, rates of other contracts would be blended so there would be no additional burden to the industry except for the costs of changing actuarial tables and forms. The changeover would be measured in millions rather than the billions of dollars the current version of the bill contemplates. He declined to say when the markup would occur.

There appeared to be little spirit of compromise at the hearing as the insurance industry and civil rights and women's groups stuck to their previously stated positions. Going against the grain, Barbara Gibbons of Concerned Women for America testified against the bill on the grounds it was too simplistic and should only apply to pension inequities, not all forms of insurance.

However, Elizabeth S. Morrison, a Baltimore insurance broker, testified in favor. She noted that several major insurers have already begun to offer unisex rates to professional women. For example, a 30-year-old attorney buying $3,000 a month disability insurance saved herself $1,000 a year, or almost 50 percent of previous cost, when her carrier switched to unisex rates.