Sen. Alan Cranston (D-Calif.) got a load of unwanted publicity last week, when a report from the Senate Rules Committee identified him as the biggest distributor of postage-free franked mail in Congress during the past summer.

Cranston immediately cried foul, pointing out that he also has more constituents than any other senator except his California colleague Pete Wilson. He had a point, because a recalculation of franked mail costs on a per-capita basis of state population showed Cranston ranked no higher than 14th on the Senate's list of postal big spenders.

But the same ranking confirms what most reporters have long suspected -- that franked mail is one more of many tools that incumbents use, directly or indirectly, to help gain reelection.

Nine of the 13 who outspent Cranston on a per- capita basis are, like him, running for reelection next year. The inclination will be to say that such subsidized electioneering should be stopped. A more rational response would be to compensate challengers for this and other incumbent advantages.

This is the first report on individual members' franked mail either house of Congress has issued, and it came out largely because retiring Sen. Charles McC. Mathias (R-Md.) was dogged in insisting on its release. Mathias, the chairman of the Rules Committee, can become indignant about the escalating cost of these mailings, but how much is too much?

The highest per-capita cost -- 19 cents -- was assigned to Sen. Christopher Dodd (D-Conn.). Is it worth that much to get a report on his legislative activities for three months? Some may think not. Many may feel that television, radio and newspapers tell them at least as much as they need to know about their senator -- and maybe more.

But ask yourself: Did the decisions Dodd and Cranston and the other senators made during those three months affect the livelihood and well-being of the average household in their states to the extent of 60 or 70 cents? The answer has to be yes, and many times over.

The problem with the controversy over franked mailings and all the other costs of what came to be called, a few years back, "the billion-dollar Congress" is that there is no way to separate the costs of representation from the implicit subsidies to campaigns.

Millions are spent each year to staff and equip district offices for senators and representatives. Such year-round operations obviously give the incumbents an advantage in their next campaigns. But those offices also dispense vital help and service to constituents who have legitimate problems with the federal government.

All of these costs ought to be made public. The constituents of Cranston, Dodd and all the others should know that the mail they get from them is not free, but has a specific price tag that they, the constituents, are paying with their taxes.

If it is excessive, the voters will surely let those members know. But an important part of their job is to communicate with their constituents. Communication costs money. Someone is going to pay, and taxpayers are the ones who should pay for the representation they get.

The way to combat the campaign benefit that incumbents get from such communications is not to curb their mail, but to assist the funding for challengers. That is something Congress has steadfastly refused to do. Congress likes the present system, where incumbents get the benefits of taxpayer-subsidized staffs, offices, mailings and travel, plus about $7 of every $10 of private campaign contributions from political action committees. Congress thinks it wonderful to grab these benefits for its own members and allow the challengers to enjoy the character-building experience of fending for themselves.

Instead of fussing about the amount of mail Cranston and Dodd and the other incumbents send out, a sensible policy would offer their challengers subsidized mailings of their own, or some other form of compensatory help, when campaign time comes.

The notion of providing such assistance to challengers has never been very high among the campaign "reforms" Congress has considered. Members of Congress prefer to argue about limits on contributions or spending. After they have debated such limits and decided (correctly) that most of them are impractical, undesirable or unconstitutional, they retire with a sense of satisfied virtue.

The spirit of Christmas is that those who have, give. The spirit of Congress is that those who have, keep the advantages for themselves.

Let's not whip ourselves into a lather trying to reduce the mail from Congress. Let's help the challengers spread the word that there may be another side to the story.