Betty J. DeVito, a mother of seven from Mercer, Pa., has been a glutton for Democratic punishments ever since she was 13 and heard Harry Truman speak from the caboose of a campaign train. s

The uniquely American affliction she caught from Give-'em-Hell Harry that day is what has compelled her this week, as a Carter delegate, to get up every morning at 6:30 to take a telephone call from the hometown newspaper, then, after a quick breakfast, head for a rules committee meeting, of a union meeting, or a state caucus, or one of the other political confabs that fill the time until she heads for the main hall at Madison Square Garden, where she stays until late at night.

Her feet have swollen to three times their normal size, she said, pointing down at her sensible leather moccasins.

She figures the week has cost her about $700, and besides that, she took a week off without pay from her job as an accountant for the state.

"People think I'm here just having a wonderful time," she said.

The sophisticated wisdom these days is that the convention is an obsolete bore where nothing of much substance happens.

But to the insurance salesman, teachers, housewives, bankers and others who populate this arena the nominating convention remains an American ritual, and it is their porthole to history.

While the cameras focus most on the strident and offbeat elements of the Democratic Party, most of the people here consider themselves mainstream Americans, privileged to be here representing their neighbors and families back home. They salute the flag, sing the national anthem, pledge allegiance with their hands over their hearts, get teary-eyed over a ringing phrase, and scream, cheer and wave placards with ceremonial fervor.

"A convention isn't fun," DeVito said. "If you're going to accept the responsibility of coming to the convention and representing people, you have to accept your duty.

"This whole row is my responsibility," she said, gesturing along the bright red folding chairs of her delegation. "If you talk to them, I think you'll find they are the best informed delegates in the hall as far as the president's stand on issues. That's my job."

Married to a steelworker, she said she has been active in Democratic politics for 27 years, and had held numerous party offices.

Every since she heard Truman talk about the importance of the voting booth, she said, "I believed in the party."

John W. Simon, a state representative from Lewiston, Maine, a Carter alternate, said of the convention's mixed reviews, "In Maine, we like hockey. I can imagine somebody tuning in to a game and thinking it's boring. But for those of us 'down on the ice,' it's anything but boring -- even if you're not Bobby Orr."

The Maine delegation had been split 50-50 between Carter and Kennedy, he noted. "There were difficult times. But this morning, at our caucus, a Carter delegate pinned a Carter pin on our govenor [a Kennedy supporter], and we all rose to our feet and applauded."

For those who are really involved in it, Simon said, the convention has been an emotional roller coaster. "There have been spine-tingling moments, like the [open convention] rule vote, the MX missile debate, the Kennedy speech," he said.

Even though he is a Carter man, he said, "I got this frog in my throat cheering for Kennedy, not Carter."

The night after the high-voltage Kennedy speech, the convention's collective spirit seemed to withdraw into a valley of listlessness. This was deemed by many to be just another clear sign of minimal enthusiasm for Carter. d

"The emotion of the Kennedy speech had drained people." Simon said, but he added, "Even if Carter doesn't have people leaping to their feet, he'll do a job that everyone can identify with."

Kennedy delegate DeKarlo Brooks was still glum as he wondered around the hall tonight. "Last night was really depressing for me," said Brooks, an accountant from Sheltenham Township, Pa.

"As a Kennedy delegate, I feel my commitment from my heart. But tomorrow has to go on," he said.

Although Brooks was not planning to desert the convention or the party, he was not quite reconciled to supporting Carter, he said. On his lapel, along with the array of blue Kennedy buttons, was one that said, "You don't have to be Jewish to vote against Carter."

The suggestions made this week that the convention delegates have been turned into robots are fighting words to democrats like Mildred McMillon, a homemaker from Tonganoxie, Kan.

"I voted my own convictions, and paid my own expenses. There's been nobody here to tell me how I had to vote on anything," she said.

McMillon was one of those people wearing outlandish hats on the convention floor. "I made it," she said. "My three grandchildren wanted me to do it, so if I was shown on camera they'd recognize me."

The hat was a styrofoam boater with a wooden donkey she said she'd been saving for such a purpose, dozens of peanut pins that she'd shellacked and given little glued-on eyes, a yellow felt Kansas sunflower, a flurry of red, white and blue ribbon and the word VOTE!

"For me, this summer has been anything but boring," she summed up. "I think there has been a lot of expression here of people's needs and feelings. I just think a lot has come out of this convention. . . . It makes you feel good to be here when history is being made."

Dee Arps, who runs a contracting business with her husband, traveled here from the Big Horn Mountain range of Wyoming. She sat attentively with her fellow delegates through the most excruciating, least dramatic, details of the platform proceedings, even when many of the delegates had deserted the hall for the excitements of Manhattan.

"We feel it's a privilege to come, even though it's rather expensive," she said. "We're so isolated in Wyoming, you know, when you come here, we really feel we're helping make history."

Mary Jo Cannon, a small brown-haired woman from Memphis, seemed as absorbed in the long afternoons of speeches and reports as some people get over their favorite soap operas.

"I'm very much concerned about several issues, especially the nuclear disposal sites," she said.

Her serious demeanor was difficult to reconcile with the outlandish hat and armor plating of buttons she wore. The hat was a sort of effigy of Jimmy Carter made of papier-mache in the shape of a peanut, with some smiling lips and teeth made of red and white socks.

She said the hat was made for her by a student in the adult education classes she teaches. And she considers herself a kind of emissary for those students, who, after all, "helped elect me and send me here."

"For me, this won't be over until I get back to school Friday night and tell my class all about it."