The U.S. Open is a little bit about golf and a lot about enduring the most capricious and unrelenting adversity. That's why it's no surprise to find John Mahaffey tied for the lead at the midpoint of this 83rd renewal of golf's four-day celebration of athletic sadomasochism. And that's why it's appropriate that Calvin Peete should be just a shot behind him.

After two rounds, only the 35-year-old Mahaffey, who has overcome everything from injuries to a broken marriage to a drinking problem, and PGA Tour rookie Joey Rassett are one under par (141) at viciously difficult Oakmont Country Club.

Mahaffey (72) and Rassett (69) find themselves a stroke ahead of three illustrious gents at even par: defending champion Tom Watson (70), 40-year-old Ray Floyd (70) and Peete. This Open should not lack for weekend marquee appeal since the fellows tied for sixth place at 143, just two strokes off the pace, are the glamorous Spaniard Seve Ballesteros (74) and brilliant sophomore Hal Sutton (74), who is second on the PGA Tour money list.

Actually, Peete, the hottest player on the course, completed only 17 holes today. He was one of 38 who couldn't finish their rounds before dark, due to a 152-minute rain delay in midafternoon when torrential thunderstorms and hail swamped Oakmont. Two spectators near the second green were struck by lightning; both were taken to a hospital for treatment of superficial burns and one was hospitalized overnight. Both were reported in satisfactory condition.

The same could not be said for many golfers in this field of 156 as scores remained astronomical. Only Peete, who was four under par for the day, showed any real mastery of the layout and even he finished in a funk as a six-foot birdie putt on the 17th hole did a 360-degree lipout and came back at him in the gloaming.

Like the 37 other golfers stranded on the course at sundown, Peete faced the displeasure and strategic disadvantage of returning to Oakmont at 7 a.m. on Saturday to finish his second round before playing his third later in the day.

"I thought we got a break. The rain softened the greens so we could shoot for the flags," said Peete, who could become the first black to win one of the four major golf championships. Because of that tour nemesis--slow play--Peete's threesome, the first off No. 1 tee after the rain delay, couldn't finish, despite being on the course 4 hours 45 minutes. "I'd really rather not come out at 7 o'clock to play just one hole," said Peete.

Such aggravations, however, are just "par" for this infuriating and borderline-unfair course. For instance, first-round coleader Bob Murphy followed his 69 with an 81. Bobby Clampett had an 82. Johnny Miller, who won the 1973 Open here, stands at 78-76--154 and probably won't make the cut. Even those who think they are playing miserably still find themselves in the hunt. For instance, Jack Nicklaus, who explored most of Western Pennsylvania this evening, ending up at 74--147, is only six strokes out of the lead and, when the second round is official, shouldn't have more than 15 players ahead of him.

Seldom has an Open offered so many fascinating contenders who are peeking over the shoulders of the leaders.

* Watson wants to become the first back-to-back Open champion since Ben Hogan in 1950-1951. He also wants to atone for blowing the 1978 PGA here, which he lost to Mahaffey in a three-way playoff that included Jerry Pate. "I'd be lying if I said I didn't have any compulsion to make up for '78," said Watson. "I played better today. I made some very good swings, I made five birdies and I sank a 50-footer. Those things help your confidence . . . I like the feeling that I have."

* Floyd, No. 2 on the money list each of the last two years, wants to cap what has been a marvelous late-blossoming career by a man who says he was more interested in a good time than a good score for more than a decade on tour. "I don't have a lotta years left to win an Open," said Floyd, who, in 21 seasons has only two victories in majors, the '76 Masters and the '82 PGA. "So I guess I'm just going to have to grab destiny by the throat and shake it."

* Ballesteros who, at 26, already has three victories in majors, wants to lay claim to the throne as current king of golf, a distinction shakily held by Watson. Should he win here, Ballesteros, the '83 Masters champ, also would be halfway to the almost unthinkable Grand Slam. "Overall, I am very happy. A 74 is not too bad," said Ballesteros whose one-iron tee shots were sprayed scattergun style as he battled marvelously to shoot three-over-par 38 on the back nine. "This year, I made the cut," kidded Ballesteros, never better than awful in his five previous Opens. "Now, I must stay in position (in the third round), so I have a chance to win on the last day."

* Sutton, 24, was the PGA's rookie of the year when he won $237,434 in 1982. This season, he's already captured the TPC. The son of a multimillionaire, Sutton's only golfing goal is greatness. He and Ballesteros are probably the world's two most consistently powerful players, through the whole bag. Should Sutton win here, the public will have to get used to the idea that golf may have a fairly decent imitation of a "new" Nicklaus.

* The soap-opera appeal of a Peete victory in the Open would transcend the narrow world of golf and become an instant national fable. Hollywood wouldn't dare dream up a tale as preposterous as the rise of this 39-year-old who was one of 19 children, grew up in Florida share-cropping poverty, never touched a club until he was 23 and has overcome the handicap of a bent left elbow.

Nonetheless, the emblem of this day was the gritty Mahaffey, a fellow so mentally tough that he may yet undo all these gaudy story lines and impose one better to his liking.

During his career, the blond 140-pounder has torn or twisted just about all the tendons in his left elbow and wrist. He has a habit of refusing not to swing at balls inbedded in jungle lies. The tough Texan has, in the last 18 months, overcome a divorce, remarried, and licked the drinking problem that contributed to the wreck of his first marriage. He's also lost 25 pounds, reworked his golf swing and rediscovered his competitive desire.

On top of all that, Mahaffey has drastically changed his attitude on and off the golf course. Always tart of tongue, Mahaffey now seems to blend his sharp wit with genuine good spirits. "I'm a lot less uptight than I used to be."

Complaining of his driving, Mahaffey said, "I'd switch to a three-wood, but I brought two of 'em and I can't hit either one."

That's the sort of grit that Oakmont demands. By Sunday night, when many a man's gumption gauge has reached empty, just a tiny dose of that kind of cavalier composure may be worth an incalculable price.

Of the two Washington-area players, Gary Marlowe with an 82--157 will miss the cut. Donnie Hammond is two over par through 12 holes.