DID YOU KNOW that Sen. Alan Cranston (D- Calif.) spent $1.6 million in mail sent out under the frank -- Congress's free mailing provision -- during July, August and September this year? Probably not, since the figures were made public for the first time Monday, thanks to a rule change sought by Sen. Charles Mathias (R-Md.). Some senators are clearly embarrassed. Mr. Cranston, for one, has some ready excuses. He represents a large state, he says, and ranks only "14th in the Senate on per constituent mailing costs." The 13 senators who spend more than Mr. Cranston on a per capita basis spent "nearly twice as much money as I" -- Mr. Cranston saves us the trouble of calculating: it's $3,084,827 -- "to communicate with roughly the same number of constituents."

He might have added that it's hard to separate the wheat of useful communication between an elected official and his constituents from the chaff of pre-election puffery. But keep in mind that personal letters to constituents and committee business account for less than 10 percent of the cost of congressional mail. That cost rose from $43 million in 1979 to an estimated $144 million this year. Most of the cost goes for unsolicited mass mailings and newsletters. There's something to be said for letting public officials communicate directly with constituents. But it's also clear that the congressional mail budget pays for a lot more chaff than wheat.

This is true even though the law regulates the amount of material sent out under the frank during the months just before elections and the content of this franked mail. The idea is to prohibit blatant electioneering. But it's also clear that the frank remains a political asset, used shamelessly by incumbents of all political stripes, from the liberal-minded Mr. Cranston to the otherwise tight-fisted Sen. Charles Grassley (R-Iowa). Heavy use of franked mail gives incumbents an unfair advantage and puts challengers at an unfair disadvantage.

Two things can be done about this. One is to reduce the volume of congressional mail. We hope that the airing of the amounts spent by senators even in this nonelection year helps bring that about. The second thing is to make sure, now that Congress has suddenly been seized by a desire to do something about the way campaigns are financed, that any changes reduce rather than increase incumbents' unfair advantage. That means when you start fiddling with the campaign finance laws, not to reduce significantly the amount of money candidates, especially challengers, can raise or spend; it may very well mean raising the contribution limits if only to make up for their erosion by inflation since they were set in the early 1970s. The problem with campaign finance today is not that too much money flows through the system, but the way it flows. Incumbents should not be allowed to vote in a system that, accidentally or on purpose, cuts off the flow of money to challengers and cements in the unfair advantage now enjoyed by Mr. Cranston and his frank-happy colleagues.