SUPPOSE YOU were put in charge of spending all the federal income tax money paid next year by a city of 330,000. If the citizens of this hypothetical city lived in families of four, each with an income of $20,000, their taxes would produce a fund of $144 million. You could use that money to run the entire Peace Corps and still have $20 million left over. Or you could run the National Institute on Aging, the Bureau of Reclamation or all the federal bankruptcy courts. In each case you would have more than enough money in the fund. It's a big sum, and Sen. Charles Mathias, in a floor speech last week, used these examples to emphasize the enormous cost of another federal budget item that is expected to be $144 million next year: congressional mail.

How can members possibly be spending so much? In 1979, they spent only $43 million to write home. Are constituents bombarding them with letters about pending legislation or requests for pamphlets from the Department of Agriculture? Has there been an upsurge of casework about lost Social Security checks or petitions for new federal highways? Not at all. Answering the incoming mail is not the problem. Personal letters to constituents account for only 3 or 4 percent of the mail costs and committee business a similar amount. All the other items mailed at the House and Senate post offices are newsletters sent, unsolicited and at taxpayer expense, to constituents.

It's easy to understand why legislators like this little perk. They get to compose the newsletter and they are invariably the heroes of the events reported. The analysis of issues is theirs and the photographs always show their best side. Some claim that the newsletters are absolutely necessary to "educate" the people of the district or to provide vital information that home-town newspapers don't find quite so exciting. But the most important benefit of a newsletter is the obvious, though in- frequently conceded one, of allowing a member to establish himself in the mind of the constitu- ent as the congressman or the senator. Over the years, this gives the member a considerable political advantage over less well-known potential opponents.

A few senators don't send newsletters at all, but another was responsible for 10 percent of the costs all by himself. The number of mailings bears no relationship to the size of the state represented. Some members simply send out far too many newsletters. Sen. Mathias, who as chairman of the Rules and Administration Committee speaks with special authority on this question, urges his colleagues to restrict severely the amount of unsolicited mass mailings they send out. "I would have to think a long, long time," he says, "to come up with a more embarrassing example of the waste of taxpayers' money." He's right.