Pro golf already has a seniors' tour for the fellows over 50, and now they're gearing up for an over-40 tour with $4 million bankrolled for next year. They'll play tournaments at $200,000 a pop, with 50 players invited. Of the over-40 set, only 13 men earned more than $22,000 on the regular tour last year.

"It's tough," said an executive type explaining all this, "for guys 40 to make a living."

Jack Nicklaus, 42, laughed. "Why'd you look at me?"

This dialogue took place two weeks ago when the papers were full of stuff implying--again--that Nicklaus is over the hill. The doom sayers pointed out that the great one was not a factor at Augusta, and he missed a cut somewhere after that, and (surest sign) he couldn't putt a marble into a manhole.

So there we were at Nicklaus' Muirfield Village Golf Club in Columbus, Ohio, asking Jack kind of quasiseriously what he thought of the over-40 tour for those fellows in the rough of middle age. Jack being a nice guy, he did not bury a wedge in any questioner's haircut. Instead, he gave us every hint that he would win again. And soon.

"As long as I can get irritated with myself, I'll be a reasonable competitor," he said. Nicklaus' idea of reasonable, of course, is another fellow's dream of immortal. "When I say the heck with it, I'll cease to be a serious competitor."

In glad news for golf, Jack's irritation in the extreme already has paid off. He won last week's Colonial Invitational. And now he will work on great golf courses preceding next month's U.S. Open at Pebble Beach. Besides his own tournament, the Memorial, Nicklaus will play the Kemper Open at Congressional the next week, May 31-June 6.

"I'm not playing badly at all, I've just had problems scoring," Nicklaus said a fortnight ago. "I played too many times in rain-delayed tournaments and just haven't putted well. I guess I'm not a good rain-delay player. I haven't been able to get my mind organized."

Two second-place finishes and a third this spring meant nothing to a guy who now has won 69 pro tournaments. To hear Nicklaus, you'd think the poor guy hadn't made a putt since Arnold Palmer invented golf. "I'd consider any change in my putting--do you have a suggestion?" he asked a newspaperman who hadn't made a putt in two decades.

Across the land, as called for by biennial custom, we read the standard Nicklaus golfing obituary. When the best player ever goes a couple of years without winning, as Nicklaus had before Fort Worth, trend-spotters declare him dead as a Podo in a pond. Frank Beard, the tour's leading money winner in 1969, said in a book that year that Jack was washed up.

Since 1969, Nicklaus has won two U.S. Opens, two Masters, four PGAs and two British Opens, along with $2.5 million. Yet another prognosis of doom was delivered in 1980 when Nicklaus hadn't won in almost two years and missed the cut the week before the U.S. Open.

That, remember, was the Open he won at Baltusrol, and he won the PGA a month later, and . . . you get the idea. So whatever scoring problem Nicklaus had this spring is hardly evidence of The End.

Look, if Jack Nicklaus says his eight-iron can do the fox trot, you better clear the dance floor. Anything Jack wants, he gets. And right now he wants to go on being immortal. Nicklaus at 42 hears footsteps.

"Somebody is going to break my major record somewhere along the line, and I'd certainly like it be more than 19," he said. Starting with two U.S. Amateur championships, Nicklaus has added five Masters, five PGAs, four U.S. Opens and three British Opens.

To that end two years ago, Nicklaus made a bold change in his game. Always the most upright of swingers, he discovered with advancing age that he wasn't hitting the ball squarely anymore. To compensate, he made his swing flatter.

This is so complicated Jack used 5,000 words to explain it in Golf Digest last month. It speaks volumes about Nicklaus that at 40 with 17 majors he yet felt the flame of competition so intensely that he literally rebuilt a swing that had worked right nicely for 30 years.

"I'm not 25 any more; I can't hit the ball 400 yards," Nicklaus said. "You change things as you go through life, don't you? I change the answers I give my wife. If your golf game doesn't change, you could be in trouble.

"I changed my whole swing plane and my chipping game. If I want to continue to be a competitive golfer, I have to change."

After 30 years of one swing, wasn't it difficult to change?

"The hardest thing to do at 40 is convince yourself changes are necessary, but if you do that, and I did, they are easy to change. I've got enough confidence in my ability as an athlete to change things rather than stay put," he said.

The world is full of golfers who have changed their games, having found the secret, the key, the one move that will make them safe against a nervous breakdown--only to discover that three minutes later, the secret doesn't work. Did Nicklaus go through such doubts?

"Sure. I'd say, 'I can't do this . . . I can't do that . . . That's ridiculous.' "

How long did it take to get comfortable?

"A long time. Three months, probably."

Three decades it takes us. Three months it takes him. The immortals are different from you and me. CAPTION: Picture 1, Colonial triumph ends Nicklaus' victory drought.; Picture 2, Besides the trophy, Jack Nicklaus won $63,000 in Colonial. AP photos