SOCIAL SECURITY is tottering. Presidential committees are frantically trying to find $200 billion to keep it going for another seven years. President Reagan, who would have radically cut back the whole vast system in 1981 only to have half the country roar in outrage, is nervous. It is becoming a national battle cry. Democrats say Republicans want to kill it. Republicans say Democrats are politicizing it. Washington is terrified. Young people say they'll never get their money. Old people wring their hands . . .

And Frank Bane, the father of Social Security: What does Frank Bane say?

"I made a mistake."

That's what Frank Bane says. He was the man who wrote the book: executive director of the Federal Social Security Board from its inception in 1935 until 1938, the man Harold Stassen once called "the best administrator in the country," a giant in the federal bureaucracy who sometimes dined at the White House but was virtually unknown to the public all his long career.

Bane's "mistake" was in failing to foresee that Americans would live as long as they do now, receiving Social Security checks for years after they turn 65. In 1930 American life expectancy at birth was 59.7 years. Though he knew it was steadily growing (it had been 47.3 in 1900 and would rise to 62.9 in 1940 and 68.2 in 1950), no one could reasonably have projected it to 73.8 in 1980.

"If I had it all to do over again," he said the other day, "I would make several changes. I believed old man Solomon. He didn't know what he was talking about. Three score years and 10? He was way off base. I would change the age situation that is, the life expectancy figure . Instead of three score years and 10, I would make it four score and 10. But I would do it very, very gradually."

Frank Bane himself will be 90 in April. He lives in an Alexandria nursing home.

To cope with the longer American lives, Bane said, one thing he would do is push back the time of retirement: move the eligibility age from 65 to 68 and the early-retirement age from 62 to 65.

"I would change it six months every year so you wouldn't lose money. I would emphasize as I do all the time with these old folk here that no one under any conceivable circumstances is going to lose a dime of what they are now, underscore now, getting."

Although he helped set up the actuarial tables, Bane suspected, even at the time, that the figures might change eventually--"We knew the problem was coming . . . in the '80s"--but the committee's urgent concern, of course, was the frightening reality of 1935.

Already then a veteran in the new and burgeoning field of public welfare, he had been Virginia's first welfare commissioner, and Tennessee's, and had served on the stopgap emergency committee that Hoover had reluctantly formed in Depression panic in 1930. Roosevelt brought Bane in after Congress passed the Social Security Act five years later.

The impact of Social Security was tremendous. Depression or not, many Americans deeply resented the very idea of public welfare. Even Roosevelt didn't advocate it when he ran for office. "Roosevelt ran more or less as a conservative, but he was scared to death by the Townsend Plan" among other things, Bane said, because the plan was an early old-age benefit program.

"People didn't believe Social Security would work" even after the payroll deductions began. Bane had to reassure them that the government would indeed pay what it had promised.

"But people were excited when the money came in. Now there's a problem of the younger people. I'm not at all worried what's going to happen to the older people. Any time anything happens to Social Security my old friends come a-running, so I say, 'Well, you're not going to lose a single solitary dime on what you are now getting.' One nice old lady who I thought was a sweet retiring maid, after I made that remark to her three times, she looked straight at me and said, 'Will you put that in writing?' " He snorted. "An economist."

Frank Bane, one of the wartime government's elite volunteer dollar-a-year men whom Drew Pearson used to call "the unknown big shot," had a reputation for handling gigantic tasks. Before and after Pearl Harbor (upon which day he was at the White House for lunch with Mrs. Roosevelt and everyone wondered why the president didn't come down), he helped work out the nation's civilian defense plan. And in the closing days of 1941 he was called upon to organize a rationing system.

The panic was on. Tires had to be rationed immediately, since the war with Japan had cut off most of our rubber supplies. Soon to follow: cars, gasoline, sugar, coffee, meat and other basic items. But a rationing operation would take six months to set up, the Office of Price Administration moaned. What to do?

Bane, then the OPA's director of field operations, spoke up. He could do it in three weeks, he said.

"The chief thing was to start," Bane says. "Before that, they always used the post offices for such things as they did and still do in Europe and Britain for programs requiring massive daily traffic in coupons , but that was too slow. I used the schools. I wired the governors and they reached the school boards . . . Kenneth Galbraith didn't like my system, said I was building a political machine."

And the outrageous Theodore Bilbo crucified him on the Senate floor because he hadn't exempted preachers. But three weeks later to the day, the rationing administration was in place, run by the locally elected citizens who oversaw their school systems, already intimately locked into American hometown family life.

Bane, it turned out, had been a Virginia school superintendent before he moved into penology and then welfare, so he understood how school boards work. His other hat, worn for 20 years until he retired in 1958, was executive director of the Council of State Governments. Based in Chicago, he commuted to Washington during the war until the day he and his wife happened to walk through the lobby of the Powhatan Hotel on their way to dinner, spotted two top federal administrators in the cocktail lounge and saw them still at it two hours later, on the way out.

"Grey said his wife's name was Lillian Greyson Hoofnagle it was foolish for me to commute. No reason I should be in Chicago particularly. She said, 'I'm protecting my property.' Those two guys, one of 'em ended up practically on relief, the other ended up selling glazed fruit."

He had known Grey from school days in Ashland, Va., and at Randolph-Macon College (A.B. 1914, varsity quarterback). During World War I when he was in Texas with the fledgling Army Air Corps and she was a Virginia school superintendent, he proposed by telegram.

"How about marrying me?" the wire said.

"Wire shock serious when shall I expect you?" she shot back.

He: "Making arrangements suit your convenience I shall be there tomorrow."

She: "Delighted you coming don't jump to conclusions."

He: "Have to give definite reason for furlough."

She: "Good chance hope you're coming."

He had told his brother, Baldwin, to join her in Virginia. When Baldwin got there, he told Grey, "I don't know what I'm doing here." She replied, "I think I'm marrying your brother tomorrow."

And when the couple emerged from the church, freshly wed, the first thing he said to her was, "Now haven't you got yourself in a helluva fix?"

They were married 35 years. She died in 1953. There are two children, five grandchildren, two great-grandchildren.

"She never did accept the proposal," he mutters. He is in the infirmary now, bedridden after a stroke, and the words come slowly. His granddaughter, Clarke Hutchinson, prompts him. They had him on pure'ed food, and he hated it. His right eye is blind, but his left doesn't need glasses. He drank sparingly, smoked a pipe all his life, he says.

He talks of the Columbia law school days, the summer job at Sing Sing ("Once I umpired a baseball game, and I learned that when they say 'Kill the ump!' they mean it") and sings the Sing Sing Song, "my alma mater." He was going to be made a colonel after the war and go to Germany to set up a rationing system. "I said to Grey, 'What kind of a colonel do you think I'll make?' and she looked me over carefully from stem to stern and said, 'You can't hold your breath that long.' "

The worst president of his lifetime, says this lifelong Democrat, was "Lyndon Johnson, easy." The worst from the standpoint of the presidency itself, not the individual, was Nixon, "but he did one thing that no other president could have done, he opened up China." The best, aside from Roosevelt, was Eisenhower, he thinks.

The stories go on. A nostalgic drive through Knoxville: There, sitting in front of his filling station, chair tipped back, hat over eyes, reading the paper, was a local character he had known in his welfare days. Bane honked. The character peered over the newspaper. "My God, Frank. I'd hoped you were dead."

Frank Bane laughs.