Members of Congress will spend an estimated $5 million of taxpayers' money this year on computers to develop mailing lists and print letters to constituents.

Hundreds of millions of letters sent at no cost to the members under congressional frank will go out next year - an election year - thanks to these computerized systems.

Under House and Senate rules, the computer lists and letters are supposed to be for "official," rather than political purposes. But for people whose business is politics, this is an impossible distinction to draw.

In a recent study on the use of computers by House members, the Library of Congress said the system "raises a plethora of legal and ethical questions . . . It is difficult to determine when a member is using computer services for official, rather than campaign, purposes."

The House provides each member with up to $12,000 yearly from his or her clerk-hire allowance to purchase outside computer services. Senators are permitted free use of the Senate computer. For both houses, comptuers have become another of the perquisites that give incumbents an advantage over any potential political challenger.

"The computer provides an effective mail-response device," a veteran House campaign aide said recently."As long as you don't say 'vote for me' on a letter it produces, anything you write is okay . . . And the payoff at election time is fantastic."

Before computers, a legislator had a difficult time keeping up with incoming mail and little opportunity to develop his or her own mailing list, beyond those people who contacted their representative.

Nowadays, thing are very different.

Take the way in which freshman Rep. Cecil Heftel (D-Hawaii) chose to develop his mail and telephone contacts back home.

In May, Heftel paid $1,356.67 from his computer allowance, according to an aide, to purchase a printout of registered voters in his district and their addresses and phone numbers. "He wanted to sit down and call people at random . . . to get John Q. Public's ideas." the aide said.

At the same time, Heftel wanted to create mailing lists of individuals with specialized legislative interests. Waiting for constituents to write him would take too long.

To help create the lists, Heftel again used his computer allowance, this time to hire - at $1,000 a month - Robert Francis Jones & Associates, a Washington-area firm that specializes in creating computerized direct-mail programs.

Jones has used with other congressional clients - a postal patron questionnaire that sought responses on particular issues.

Included in the Heftel material, which was sent to all 133,000 postal addresses in the Hawaii first congressional district, was a pre-paid return envelope.

Between mid-May and November, four such questionnaires went out. The first was on saccharin, the second on energy, then Saturday mail delivery (preceding a hearing on the subject in Hawaii) and the last one on the supersonic transport and the Panama Canal.

With each mailing the return has increased. THe first drew 14,000 responses, the second 15,000, and the third 17,000 and the last is expected to top 18,000.

Jones coded the returns and filed them on computer tapes. Out of the first 9,000 responses, only 1,000 were repeats.

At the end of the process Heftel will have the responses of a significant number of his constituents on tape and he plans follow-up letters in each area of their interest.

Handling the cost of such an effort shows the varied nature of the operation. Official allowances paid for printing: the frank covered the mailing: the computer work came out of Heftel's computer allowance, but the pre-paid postage for the return letters was paid by Heftel personally, according to an aide.

A millionaire, Heftel put more than $500,000 of his own money into his successful 1976 House campaign. Using his money to develop the computer mailing list was his idea, an aide said.

Under House rules, a member can later purchase his or her mailing list for use in a campaign as long as the member reimburses the Treasury for the costs involved.

The Library of Congress study said that "although such campaign use may not result in a violation of . . . statues or regulations, certain ethical campaign use which a member may make use of material prepared with official funds."

According to the aide, Heftel will restrict his use of the lists to sending information on legislation to constituents. A House Administration Committee aide said recently, however, that several members have bought their officially produced lists for later campaign purposes.

The Senate has a different computer setup in which members get free use of the Senate's computer facilities. The benefits, however, are the same.

Freshman Sen. John H. Chafee (R-R.I.), like Heftel, came to office without a constituent mailing list. The statewide list he had from his campaign was "politically targeted," according to an aide.

For his first newsletter, Chafee wanted to reach everyone in Rhode Island, not just supporters. After a search, his staff came up with a list of 209,000 households in the state. The list was in the possession of the Republican National Committee.

According to a GOP committee aide, the list was put together by the RNC in 1975-76 for a presidential election phone bank operation. It has no political identification attached to the names on the tape, but addresses and phone numbers were listed.

Chafee wrote a $1,700 check against his consolidated official Senate expense allowance and sent it to the RNC to pay for the list.

In late September, however, the Senate Rules Committee, which supervises the S computer and the use of allowances, questioned whether a political party list could be put on the computer.

Under new Senate rules, names with political designations are not allowed on the computer, since it is to be used for official business only.

The matter is pending, and Chafee's newsletter has not been sent.

Other senators use their allowances to purchase mailing lists and hire consultants. Jones, for example, had eight Senate clients from 1973 to 1976.

An exhibit placed in the record of a Common Cause lawsuit aimed at cutting down use of the congressional frank illustrated how at least one consultant proposed that the Senate computer be used.

In a 1972 direct-mail plan given an unidentified senator up for re-election, Derry Daly, the consultant, suggested placing in the Senate computer a target list of "blue-collar" residents in 21 counties where the senator was weakest.

He also proposed an August mailing to those individuals (a "Dear Resident' letter on (the senator's) stance on prohibiting TV broadcasting program") that would be [WORD ILLEGIBLE] stuffed in (the) Senate" computer . . .

Since the "blue-collar" lists was [WORD ILLEGIBLE] political and the letter was about one of the senator's speeches, it would quality as legitimate constituent communication under current rules. Nonetheless, as the consultant's proposal shows, it was designed as an integrated element in his re-election effort.

The Senate also has developed a second level of computer mail support that promises future political benefits, as well as immediate official ones.

Called the Computer management Service, it provides senators with electronic handling of incoming mail and supplements that with the ability to file, retrieve and follow up on letters with a speed and ease not available before.

Under the system, incoming letters are reviewed, and pre-written paragraphs, filed with the computer, are listed for a reply. The material is punched into the computer and the return letters are ready the next day.

Each paragraph - one senator's office said he had 700 on his computer tape - is coded by subject. At any later date, the name of an individual who received a specific paragraph could be pulled from the computer and that person could be sent a follow-up letter.

The cost of the system is expected to run close to $47,000 for each senator using it. But as one aide put it, the service is as good as hiring three or more staff members.