Someday, Jack Nicklaus will shrink to mortal size, the hero become merely human.

But not yet.

Someday, hard though it is to believe now, he will no longer strike the flint of his will against the steel of his skill and make sparks that set a golf course ablaze with his praise. No longer will he, in a twinkling, birdie seven out of eight holes to brand a course record at a famous old club, as he did yesterday with seven-under-par 65 at Congressional Country Club.

Someday, Nicklaus will no longer be able to lap the field with stunning speed as he moves from "off the board" to the splendid solitude of first place in the span of only two hours, as he also did yesterday at the Kemper Open.

Nicklaus was seven shots behind when he teed off, but finished this second round at 72-65 -- 137 with a two-shot lead over defending champion Craig Stadler (72-67 -- 139) and a three-shot margin over Gill Morgan (68-72 -- 140). Behind the Bear, the Walrus and the Doctor on this event's glamorous leader board are Seve Ballesteros (69), George Burns (66) and Gavin Levenson (73) at 141.

Someday, that look of almost frighteningly deep concentration and implacable determination will cross Nicklaus' face and nothing will happen. Birdie putt after birdie putt will not drop into the very heart of the cup as huge galleries slap their foreheads and howl in delight.

But those days have not yet arrived. Not yet, not while this 42-year-old Bear can still send his growl of birdies though the woods, scattering the smaller animals of the golf kingdom before him.

Nicklaus was in his glory again this calm, misty, cool afternoon at Congressional as -- in an atmosphere of gathering pandemonium and disbelief -- he birdied the third, fourth, fifth, seventh, eighth, ninth and tenth holes. Except for an uphill 15-foot putt at the easy par-5 sixth, which he missed by less than a cup's width, Nicklaus might have had eight consecutive birdies.

"I haven't had a streak like that in a while . . . I didn't think I was quite capable of 65 on this course," Nicklaus said with a grin. "I didn't want anybody to talk to me. I just didn't want to wake up."

Nicklaus shot a surrealistic 30 on the (harder) front nine, the lowest nine-hole total in Congressional's history. His 65 on what is now a par-72 layout also made him the first player to score seven-under-par here. Tommy Jacobs had a six-under 64 in the '64 U.S. Open when par was 70.

Perhaps only Nicklaus the perfectionist could say of such a round, "It was a pretty good round of golf . . . but, when you're seven under after 10 holes, and you hit all of the last eight greens (in regulation), you expect to make some more (birdies). But I couldn't get any more of 'em in, although I thought I made some awfully good putts. But I obviously shouldn't complain."

Nicklaus' shot-by-shot recitation of his round had a certain hypnotic quality as, feigning a bored drone, he would say, "Next hole, I hit driver, five-iron to about five feet . . . (pause) . . . and made it. Next hole, hit a driver and a nine-iron to 18 inches (pause) . . . and made it."

Finally, after going through his birdie putts of 10, 5, 1 1/2, 6, 18, 9 and 12 feet, Nicklaus reached his final subpar hole -- the 10th -- and said, sardonically, "Thus endeth the round."

For those who followed this noble pilgrim's progress, there was no boredom in his performance. When Nicklaus started the day with a sand-save from the trap at No. 1, his large gallery hardly suspected it would be the last green he would miss all day.

When Nicklaus, who won three weeks ago at the Colonial in Fort Worth, Tex., hit a two-iron and a five-iron shot to within the shadow of the flag at the tough par-4 third and fourth holes (453 and 420 yards) to set up birdies, a buzz was running over the course.

Many fans had been watching the back-nine collapses of Levenson, who had reached seven under par before finishing bogey, par, double bogey and bogey, and Calvin Peete, who got to minus six before crumbling to 41 on the back for 143.

Now, they were racing to the remote fifth hole to catch Nicklaus. He didn't let down his fast-arriving herd, as his nine-iron shot at the 409-yard hole stopped a few turns short of hitting the stick, leaving him a tap-in birdie.

That's when the streak began to get spooky. At the 166-yard seventh, where Nicklaus had lipped out a possible hole-in-one Thursday, he blanketed the stick again, then rolled in a six-footer.

At the eighth, Nicklaus forgot to clean the mud from his spikes, sliced his drive into a fairway trap, then grumbled to his caddy -- son Steve -- about his lost footing. In the trap, Nicklaus backed off his nine-iron shot ("felt myself tensing up . . . no time for that"). On second try, he feathered his most deft shot of the day to 18 feet, then ran down a big left-to-right breaker that he knew was on the track as soon as he touched it.

At the ninth and 10th, both par fives, Nicklaus drove into the rough, but with his legendary powerful game out of tall grass, hit excellent safe second-shot layups, then wedged to comfortable birdie range.

So immune to the damp rough was Nicklaus that he didn't even know, until told, that at one stage he'd missed six fairways in a row. Those last two birdies got the fist-pumping treatment as longtime Nicklaus watchers began searching for comparable birdie binges, like his first-round 63 at Lower Baltusrol in the '80 Open and a third-round 66 at Oakhill in Rochester, N.Y., at the '80 PGA.

As he walked down the 11th fairway, Nicklaus turned to the gallery and shrugged, as if to say, "I don't quite know how I do it, either."

Then, the magic stopped. Putts of 12, 9 and 14 feet at the 12th, 15th and 18th holes went past the edge. Now, for Nicklaus, the hard work of holding a lead will start. All year, he has shown his old knack for reaching the lead, but only once has he managed to keep it.

To be sure, there were 152 other golfers on the long, soft course yesterday, and many of them had rounds of note on a day when the stroke average was a tough 74.44 and the midpoint cut at 149 equaled the highest of the PGA season. George Archer shot 66 to tie Gibby Gilbert (72) at 142 for seventh place, and folks like Peete and Wayne Levi at 143, and Andy Bean and Lon Hinkle at even par 144 were still very much in the hunt. It only seemed as if Nicklaus was alone.

Stadler, for instance, got to six under par, and, with a birdie at the 18th, could have shot 35-30 -- 65. Instead, a wild final drive led to bogey. "Jack and I had a helluva best ball between us (61)," said Stadler with a laugh. "I really don't know what the hell is going on with my game right now. Nothing is 100 percent comfortable . . . I need a week off . . . I've just been hunting for something that will get me through the week . . . Today, it was a little draw shot with the irons that I found on the practice tee.

"I've never hit the irons better. It's an added joy, an added inspiration, to experiment with your natural (left-to-right) game and have it work when you need it."

On Saturday, if predicted rains skirt Congressional, the Kemper will have the crowd-drawing show of its dreams: a final threesome of Nicklaus, Stadler and Morgan, and Ballesteros and Burns playing directly in front of them.

However, for those with a sense of golf history, for those who will feel an acute loss when these electric afternoons of Nicklaus drama have finally passed, the rest of the '81 Kemper Open will have a hard time surpassing this fine Friday.