He is, at age 81, the dean of the Senate, an institution unto himself, a courtly old lion who simply doesn't want to quit.

The Senate has been his life for 35 years, his very reason for being. It's hard to imagine the Senate without John C. Stennis or John C. Stennis without the Senate.

"I couldn't find a stopping place. My roots go too deeply into too many matters here," Stennis explained in a deep baritone voice. "I didn't feel I should just turn around and walk off, looking for the easy life."

"It wouldn't have been easy for me," he added in his thick southern drawl. "I'd enjoy continuing to serve. I want to continue to serve, and, I feel I had a duty to offer to continue to serve."

So after 12,000 roll-call votes, eight presidents, and six successful campaigns, John Cornelius Stennis is once again offering himself to the voters of Mississippi. Early indications are that the voters once again are receptive; a Jackson Clarion-Ledger poll last weekend showed Stennis leading his Republican challenger, Haley Barbour, by 59 to 27 percent. But in Barbour, a 34-year-old lawyer and former high school football star from Yazoo City, Stennis faces something he hasn't since 1947: a serious opponent.

Barbour treats Stennis as Mississippi's most revered senior citizen, which he is. There are no philosphical differences in the race, he said. "We're two Reagan supporters, two conservatives, and two Presbyterians."

The big difference is age. Barbour, born 13 days before Stennis' first Senate election, has a pretty young wife, two children, age 7 and 2, and the campaign slogan, "A Senator for the 80s."

The slogan's implication is clear. Stennis, if reelected, would be 87 years old when his term ends; Barbour would be 40.

"This election isn't about the last 35 years," declared the challenger. "It's about the next six."

Stennis, reelected with only token opposition five times, has reacted with his first real campaign since 1947. He has hired, at the urging of Senate friends, his first campaign consultant; filmed his first TV commercial; held his first fund-raiser (he had never spent more than $5,000 on a campaign until 1976), and visited each of Mississippi's 82 counties.

"I realize I have to stir around more," Stennis said in his sedate Senate office, which overlooks the Capitol and the Supreme Court buildings. "The burden was on me to show up and come in contact with people, younger people and all."

No one in the Senate questions Stennis' integrity or contribution to the body. The possessor of a tremendous booming voice, a Phi Beta Kappa key and a universal reputation for fair-mindedness, he has long been one of its dominating figures -- a senator's senator, an adviser to presidents, a man of enormous power and influence.

But there are those, even among his friends and greatest admirers, who privately wish he would retire gracefully with his reputation intact before his years and a finicky electorate catch up with him. House Minority Whip Trent Lott (R-Miss.) chose not to run against Stennis for this very reason, telling friends, "I don't want to go down in history as the man who sent John Stennis out of the Senate."

"People keep asking me, 'Why is Stennis running?' " said one senator. "These are Democrats who are genuinely fond of him. There is a good-sized group that privately wish he won't run, and don't think he'll win."

"I certainly don't want to be around this place when I'm in my 80s, but some of these old boys just don't know when to quit," says another longtime Senate colleague. "I would have hoped he'd have had the good sense to step aside. It would be a very sad thing for a man of his quality to be carried out of here feet first."

Even when spoken from behind a cloak of anonymity, these are harsh words in a mutual self-protection society like the U.S. Senate.

To outward appearances, Stennis is unusually fit for a man his age. The lines in his face run deep, but his pale blue eyes are sharp, his mind keen. He walks with a bounce to his step, his back ramrod straight.

"My doctor tells me I have the body of a man 10 years younger," he told one interviewer.

He proved to be a man of uncommon courage and endurance when he was shot twice in a late-night holdup as he got out of his automobile in front of his Northwest Washington home in 1973.

His pancreas, a vital organ, was "slivered," as the doctors at Walter Reed Army Hospital later told him. He lost large quantities of blood and did not fully regain consciousness for weeks. One night he dreamed of reading the headline, "Stennis Dies in His Sleep." Later, he wondered whether he would ever walk again, let alone come back to the Senate.

But within 17 months Stennis, then 73, was back on the Senate floor, leading a grueling seven-day debate on a military procurement bill as chairman of the Armed Services Committee.

Stennis admirers say he retains that same endurance. When this year's military procurement bill was up in May, one aide, years his junior, recalls going onto the Senate floor with Stennis at 9 a.m. and not leaving him until 6 a.m. the next day.

When debate on the tax bill went into the wee hours of the morning last month, Stennis didn't leave the Senate until 5:30 a.m. He was back by 8:30 a.m. to keep an appointment with his barber.

"He's tougher than hell," says Sen. Henry M. Jackson (D-Wash.). "He's just different than other people. Some guys are dead around here at age 40. . . . He's alert and sharp. He remembers everything. The John Stennis story is a story of a truly vigorous man -- physically and mentally."

His friends weren't particularly surprised when Stennis announced last year that he would seek reelection, a feat accomplished by only 10 senators over 80 in history.

Since his wife of 53 years, "Miss Coy," entered a nursing home in Mississippi several years ago, the Senate has been Stennis' work and recreation, according to associates.

"John Stennis has a strong public service motivation behind him," says one senator. "At the same time he's been at this so long that he wants to keep going, even if he dies in office. This is the way he wants to go."

"Prestige or social life don't mean anything to him," adds the senator. "I think the old boy doesn't know what else he could do if he wasn't here, and he's afraid he would die if he wasn't here."

Stennis, a former circuit court judge, is an "old school senator," a classic southern gentleman in manner and outlook, the moral leader of the dwindling old bloc of southern Democrats.

Elected to fill the unexpired term of the late Sen. Theodore G. Bilbo, a renowned racist, Stennis, a fiscal conservative with a tinge of hill country populism about him, joined a Senate in 1947 that would be controlled for much of the next quarter century by southerners.

They were, for the most part, men who shared the same rural backgrounds and views on the cutting issues of the day: They opposed racial integration and big government; they supported a strong national defense.

Stennis first gained national attention as a member of the committee investigating charges against the late Sen. Joseph R. McCarthy (R-Wis.). Because of his fair-mindedness, judiciousness and decency, the Senate has turned to him during some of its darkest hours ever since.

He was the first Democrat to rise on the Senate floor against McCarthy, whom he accused of using "slush and slime" in hunting communists. In 1967, as the first chairman of the Ethics Committee, he led the investigation into misuse of campaign contributions by the late Sen. Thomas J. Dodd (D-Conn.), whom the Senate censured at Stennis' urging.

By the Vietnam war years, Stennis had become one of the Senate's most powerful members.

In part, his influence came from the seniority system, which eventually made him the second-ranking Democrat on the Appropriations Committee, and chairman of the Armed Services Committee until the Republican takeover of the Senate two years ago. In part, it came from the force of his personality, which enabled him to dominate debate, and his innate sense of decency and fair play.

It helped him bring pork-barrel projects to Mississippi. His campaign literature claims that 34,000 military and civilian workers with an annual payroll of more than $500 million are located in Mississippi because of his efforts, and that he is responsible for 35 destroyers, five Marine amphibious assault ships, and five Navy cruisers being built at the Pascagoula, Miss., shipyard.

Richard Nixon tried to take advantage of his friendship with Stennis during the Watergate scandals by proposing that the Mississippian, then recovering from his bullet wounds, authenticate the Watergate tapes rather than having Special Prosecutor Archibald Cox subpoena them. Nixon and his family pointedly referred to Stennis during that time as "Judge Stennis," a reference to his earlier career.

Like most of his early southern colleagues, Stennis was an outspoken segregationist, though never a ranting racist like Bilbo or as well known an obstructionist as his Mississippi colleague, former Sen. James O. East- land.

In 1964, he was co-author of a proposed amendment to the Civil Rights bill that would have given federal grants and loans to blacks who migrated to states with small black populations. In the early 1970s, he led the Southern assault on racial busing policies.

He was also one of the authors of the Southern Manifesto, a document signed by 101 southerners in the House and Senate, condemning the Supreme Court's 1954 school desegregation decision.

Stennis, who represents a state with a 35 percent black population, didn't support a single major piece of civil rights legislation until this year when he somewhat reluctantly voted for the extension of the Voting Rights Act. "I didn't want to go back to all the days of misunderstanding," he says. "I didn't want to turn around and go back."

The Senate is a vastly different place than when Stennis joined it. Southerners, who chaired 10 of 17 standing committees in 1951, no longer control the body. Most of Stennis' old Senate friends are dead. Jim Eastland, his old Mississippi colleague, has retired and gone back to his plantation.

Republicans control the body now. Stennis' influence has diminished. He no longer chairs the Armed Services Committee, or any subcommittee.

The work load has also changed. There were 73 roll-call votes during Stennis' first year in the Senate; last year there were 497.

Some Stennis admirers say the old senator has changed, too--that he seems alert and lively at times, but distracted and forgetful at others.

"When you get that age you tend to narrow your focus," says one member of the Armed Services Committee. "You concentrate on a few things that the committee does that you have a deep and passionate interest in. In Stennis' case this means ship construction and guarding the Reserves from any cuts. He's also very concerned about spending too much money."

The Tennessee-Tombigbee, a 232-mile water project through Mississippi, is another of Stennis' deep concerns. "The Tennessee-Tombigbee is kept alive solely by Stennis calling in his influence," says another senator.

Stennis insists he hasn't changed. He still has a daily workout. He still paces nervously across the Senate floor, like a caged lion, during debate.

As the old senator sees it, there is no reason to lay down the plow. "I still feel as good as I did when I was 60," he says. "All the signals I've had are to go, to stay on the go. Not to stop."