Mass mailings - attempts to bully Congress?

The biggest mail campaign ever to sway Congress will draw to a close as the Senate acts to abolish the 10 percent withholding tax on interest and dividend income.

''It's clear that withholding will not take place on July 1'' as scheduled, says a confident Sen. Bob Kasten, the Wisconsin Republican who has made his mark during his first term by leading the fight against withholding.

But long after the withholding measure meets with its demise, Capitol Hill will feel the effects of the methods used to defeat it.

The banking industry poured millions of dollars into an effort that virtually buried Congress in mail opposing the measure. Banks and credit unions inserted notes in monthly statements urging customers to write their legislators.

Members of Congress received boxloads of protest coupons clipped from newspaper ads bought by savings institutions. Deluged by mail, Sen. J. Bennett Johnston (D) finally bought his own advertisement in Louisiana newspapers telling constituents that he couldn't answer all of it individually.

Congressional leaders complained loudly about the pressure techniques of the banking lobby, charging that they distorted the issues. But the campaign worked. And already there are signs that other groups will try to follow the same path.

The insurance industry has launched a campaign that looks similar to the the banking effort. In full-page newspaper ads earlier this week, the two biggest trade associations of insurers argued against a proposed law to ban sex discrimination in insurance.

''Women live longer than men. Congress shouldn't penalize them because they do,'' reads the ad, asking readers to ''let your senators and representative know that you oppose'' bills requiring sex-neutral insurance.

Most insurance companies now use sex as one basis for determining rates and benefits. Men pay more for life insurance, while women pay more for medical coverage and often receive smaller pension checks. Young men pay more than young women for automobile policies.

The Committee for Fair Insurance Rates, a group formed to oppose the legislation, has sent out one of the slickest mass mailings yet seen on Capitol Hill. The mailings, sent to districts of key members of Congress, include an opposition letter already typed out, ready for the constituent to to sign and mail - with the constituent's own letterhead printed on top.

At first glance the letters don't look like mass mailings, says an aide to Sen. Russell B. Long (D) of Louisiana, who has received 2,200 letters, cards, and telegrams on the insurance bill. He says the senator began to spot them as computerized because ''they are very neatly typed, articulate, and to the point.''

Rep. James J. Florio (D) of New Jersey, chairman of the energy and commerce subcommittee that has reported out the insurance bill, calls the insurance industry campaign ''an unthoughtful effort to intimidate the Congress.''

In the Senate Commerce Committee, the lobbying effort in support for the insurance reform act appears to have made inroads. Sponsored by the panel's chairman, Sen. Bob Packwood (R) of Oregon, the bill emerged from committee last December with relative ease. But it is now running into resistance.

Meanwhile, yet another group, supporters of tuition tax credits for private schools, is gearing up for a mail campaign that might soon eclipse the insurance letters. Senator Long's office reports receiving 2,000 cards so far in support of the proposal.

One question that still hangs over the lobby groups, especially the industries, is whether Congress will get back at them with higher taxes or other unfavorable legislation.

''I don't think there is a sense of vengeance on Ways and Means,'' says an aide to Rep. Dan Rostenkowski (D) of Illinois, chairman of the Ways and Means Committee and a proponent of the tax withholding bill. ''It was a stinging defeat we were handed by the banks. But that's politics.''