To hear some tell it, he is a tired old warhorse who has pulled too many wagons too far.

While John C. Stennis may not run up courthouse steps anymore, he still has a gleam in his eye and walks at a vigorous pace in the state he has served as U.S. senator for the last 35 years.

He has hit the backroads of Mississippi hard, campaigning to bury the notion that at 81 he is too old to seek a seventh term.

One of the Deep South's last political patriarchs, he rose here one recent morning at 5:45 to pump hands, kiss babies and recharm voters. By mid-afternoon, he had delivered six speeches.

"I don't feel like the average person of 81," he said. "After I was shot [in a 1973 Washington mugging], doctors told me I had the constitution of a man 10 years younger.Well, I feel 10 years younger than that. Just figure I'm 60. I certainly don't feel any older."

Republican challenger Haley Barbour, however, has not let voters forget that Stennis is an actuarial gamble. At a cocktail party in Jackson, Barbour, 35, regaled, guests about his latest television spot starring a gray-haired old man. "He says, 'I've voted for Sen. Stennis every time. But when you get old, you should sit down, retire.'"

"Whether Stennis is too old is for the people to decide," said Barbour, whose campaign slogan, "A Senator for the 80s," takes a dig at the octogenarian incumbent. Stennis counters that Barbour is a mere pup, unfit to do "a man's job."

"Even though he called me a boy, I'm not going to call him an old man," said Barbour, an amiable Yazoo City lawyer who once helped catapult other Republicans to power as state party chairman.

He and Stennis are conservatives, Presbyterians and Reagan supporters with about $1 million to spend. Recent Stennis polls put him comfortably ahead, 61 to 27, while Barbour polls narrow the split to 54 to 36 with an 11-point difference among likely voters.

Many here think that all Barbour can do is pray for rain, fire up a high-tech media blitz, hammer at Stennis' 616 missed roll-call votes in the last six years, challenge him to debate and needle Stennis about his age. Stennis has refused to debate.

"I've never said he was senile," Barbour said."I have no idea whether he is or not. That's not good manners. I'll play hardball, but I'll never be vulgar."

But the "Happy 81st Birthday" banner, unfurled at the Neshoba County fair in August, bordered on dirty tricks, Stennis campaign manager Joe Blount said. When he tried to cut it down with a knife, he claims Barbour associates interfered. Nearby, a TV crew taping Barbour spots stalked the senator, hoping for a stumble, but he did not.

As a precaution, a flying wedge of aides hid Stennis from view. A birthday cake, boody-trapped with 81 candles, never made it to the stage.

Meanwhile, a Barbour fund-raising letter signed by Reagan was mailed to Stennis' elderly wife in a nursing home, further reminding voters of the age issue. Soon afterwards, Stennis visited the White House, presumably to remind Reagan of his past support and, it was reported, secure a promise that the president would not campaign against him.

But Reagan TV spots plugging Barbour are playing and Vice President Bush has come here to campaign for Barbour.

In a state devastated by budget cuts, Barbour appears to have little chance of getting many votes from Mississippi's poorest citizens -- blacks who account for 35 percent of the vote. Last session, Stennis backed the Voting Rights Act, the first time he has ever voted for civil rights legislation. While Barbour chides Stennis for his about-face, Barbour will not say how he would have voted.

And in a state where respect for elders is cherished, Stennis backers say the senator has stood for too much too long -- federal dollars, jobs in defense plants and shipyards and seniority in Washington. All that, they argue, is too much to chance with a "young whippersnapper."

Recently at Pearl River Junior College, Stennis was swamped by autograph-seeking students. Teresa Ginn, 19, said he will have her vote. "He reminds me of my grandfather."