The mail is the kind no member of Congress can comfortably withstand.

"Please . . ." a woman from Dallas implores. "I don't know what we would do for grandma if saccharin is banned."

"I am . . . a former fat person!" another woman, from Huntington Beach, Calif., exclaim. "I use saccharin every day in cooking."

"As the mother of a 12-year-old diabetic child, I appeal to you . . ." writes a third petitioner, from Dunwoody, Ga.

A man from Mount Juliet, Tenn., calls the Food and Drug Administration's contemplated action "preposterous, morbid, sadistic."

A woman from Cuero, Tex., sums it up: "I depend on this artificial sweetner for all my 'goodies.'"

These are random fragments from thousands of letters that have come the Rep. Paul G. Rogers (D-Fla.) in the three weeks since the FDA announced that sometime this summer it intends to ban further sale of saccharin as a food because a Canadian study found that saccharin causes bladder cancer when fed in extremely heavy doses to rats.

Rogers is a special target for the save-the-saccharin letter-writers: he is chairman of the House Commerce subcommittee on health, which has jurisdiction over FDA. But other members of Congress say they, too, have been receiving near-record amounts of nearly unanimous mail on the question.

It is too early to know how Congress will respond. But in the House, 165 of the 435 members have signed up in support of legislation by Rep. James G. Martin (R-N.C) that would relax the law and let FDA leave saccharin in use in foods, subject perhaps to such safeguards as warning labels or dosage limits.

The saccharin issue is complex. Primarily it is a health issue, but it also involves corporate profits and the national lefe-style. Diabetics are the main group affected, and they are writing most of the letters. There are an estimated 10 million diabetics in America, nearly 1 American in 20. But pudgy people have an obvious stake in the issue, too. So do the soft-drink industry, and the sweetness industry generally. Three-fourth of the saccharin consumed in this country goes into diet soft drinks, and sales of such drinks - one-eighth of all soft drinks consumed - are well above $1 billion a year.

According to the Agricuture Department, Americans last year consumed an average of 126.4 pounds each of sugar and other so-called caloric sweeteners, plus enough saccharin to produce the sweetness of eight additional pounds of sugar. (Saccharin is 300 to 400 times as sweet as sugar, experts say.)

Those consumption statistics work out to about a third of a pound of sugar or other caloric sweetner per day.

Our diet has not always been that sweet. Thre reason may simply be that sugar and other sweeteners were not so readily available in earlier times.

Karen Hess, who, with her husband, John, wrote the book "The Taste of America," sayd that the earliest recipe - 1832 - she has found for New England baked beans contains no sweetener.

The earliest she has found containing a sweetener, she says, is an 1845 recipe that notes that some bean bakers that notes that some bean bakers add a spoonful or two or molasses. Even that is "not every much in two pounds of beans," she observes, but from mid-century on, American food grew sweeter.

One important event, Hess says, was the invention of Jell-O in 1897. Another she cites was the publication of Fanny Farmer's influential cookbook. The "year after year, there were more sweet salads," Hess says.

The first year for which the Agriculture Department has per-capita caloric sweetener consumption is 1909, when the average was 87.5 pounds. It stayed about there through World War I, then rose in the 1920s to more than 120 pounds some years. Some experts think the exara sugar was going into homemade alcohol; those were Prohibition years.

Consumption declined in the 1930s and early and mid-1940s because of the Depression and World War II. It stayed fairly constant - at a little more than 110 pounds per capita - through most of the 1950s, then started edging up toward current levels.

There are various problems with the high current levels of consomption, according to food experts. Measured in calories, sugar and other sweeteners - the main other sweetener is corn syrup - now provide about a fifth of the average American's daily diet. But sweeteners contain none of the protein, vitamins and minerals the average person needs. These things must come from the other four-fifths of the diet: in this sense, the sugar fifth is wasted.

A second problem is the more familiar and simpler one, that sweetened foods are fattening. A third, related problem is the pervasiveness of sweeteners. A high percentage of our food today is processed, as opposed to fresh, and a high percentage of processed food is sweetened.

More than two-thirds of our daily sugar and other sweeteners comes to us in processed foods, including soft drinks and other processed beverages.

This pervasiveness of sweeteners is a particular problem for diabetics, and one reason saccharine is so important in their lives. We have been brought up on sweet food. "My son needs to be able to go to the corner store and order a Tab when his friends order a Coke," a mother of a diabetic wrote to Rogers. A man who is a diabetic wrote that "my work demands highspeed performance all day. In the evening, it's a bottle of diet soda. Sweeteners in these and certain dietetic desserts make life worthwile for me."

Under the so-called Delaney amendment to the food and drug laws, however, "no food additive shall be deemed to be safe if it is found to induce cancer when ingested by man or animal." The FDA thus says it had no choice in the case of saccharin, even though most experts say the likelihood of saccharin's causing cancer in a normal user is extremely low.

Cigarettes are far more likely than saccharin to cuase cancer, many of the letters to Rogers and others in Congress point out, yet the government requires only a warning label on cigarettes, which are not subject to the Delaney amendment because they are not a food.

One way out for Congress - which does not want to vote against diabetics, but does not want to vote for cancer, either - would be approval by the FDA of some other artifical sweetener in place of saccharin, now the only one cleared for sale in this country. Such sweeteners are known, and work is progressing on others, but FDA spokesmen say approval is a long way away.

So Rogers has asked the Carter administration for its recommendation on saccharin, and he and his Senate counterpart, Edward M. Kennedy (D-Mass.), have asked that further studies be made. That would give them more time to decide what to do.