The challenge confronting Ben Smith today is formidable. But, then, nobody has to tell him that. Here's a man who knows how difficult it is to win a PGA-sanctioned golf tournament. Here's a man, for that matter, who knows how difficult it is to stay ahead of bill collectors and unemployment lines.

Smith has a four-shot lead going into today's final round of the Showdown Classic. If he holds on and wins he will become the first man in history to triumph in a PGA Senior Tour event who didn't play on, and graduate from, the regular PGA Tour.In one 18-hole round today he could bust the caste system, he could shatter the odds, he could win one for the masses. It could be a huge historic moment.

Of the more than 200 official tournaments contested since the over-50 Senior Tour began in 1980, every one has been won by a Regular Tour graduate. The Senior Tour has seen only one winner, in fact, who didn't also win a Regular Tour tournament. That being Larry Mowry, who won twice last season.

Otherwise, it's always Miller Barber or Don January or Gary Player or Chi Chi or Arnie or somebody like that taking home the first place money.

The obvious deduction is that winning requires experience, confidence and the maturity that comes from playing a lifetime of tournament golf. It's one thing to survive the Senior Tour qualifying school and make cuts and even a few checks; it's another to actually be A-number-one, king-of-the-hill, top-of-the-heap.

Smith knows this only too well. Several times, he's been in a position to win as a senior since joining the tour in 1984. He led the first two rounds of the Dominion Tournament in 1985 - and then finished fifth. He was in contention at the 1985 de Maurier Championship down the stretch - and finished second. And last year he led most of the first two rounds at Seattle - before finishing sixth.

Of course, as Smith was saying yesterday after throwing a 67 at the Jeremy Ranch golf course he called "beautiful," "gorgeous," and a variety of other words too numerous and wonderful to mention, the above listed close calls could one day work to his advantage.

Like maybe today.

"You can't say I haven't paid my dues," he says. "You have to go through a lot of stages out here, and I have."

Also in Smith's favor is his past.

Now, there were a lot of times in Ben Smith's life when it didn't appear that his past was going to do him a lot of good.

He lived a decent life, there was nothing wrong that way. It's just that he had a tendency to bounce around a lot. Stability was not his middle name. To sum up his first 50 years in a nutshell, it's like this:

He joined the Marines out of high school, left the Corps and, by the time his hair had grown out, lost all his mustering-out pay playing golf. After that, in somewhat chronological order, he worked as an insurance salesman, played golf, worked as a night club operator, played golf, worked as a service station mechanic, played golf, worked as a bowling alley mechanic, played golf, and worked as a self-employed auto mechanic - and played golf..

Actually, "played golf" might not be the correct phrase, since golf was often how Smith attempted to make it from rent payment to rent payment. He was proud enough of his own game to make the occasional wager on it. He once moved from Texas to California because he heard the betting was better in California.

He never joined the PGA Tour during his first 50 years because he never came across anyone else who wanted to gamble on his game.

Whenever he ran into any lengthy losing streaks playing golf - it happens - he would have to get desperate . . . and get a real job.

"When you're playing with your own money and don't have enough in your pocket to pay off, that's real pressure," he says.

Playing on the Senior Tour, with somebody else's money, has been a dream come true.

"I've been living this dream for four years now," says Smith, who adds, "I never have any trouble sleeping."

He's already made $76,406 this season - to rank 21st on the money list - and he's made $355,274 for his Senior Tour career.

Nobody doubts he can play the game. Nobody doubts that Ben Smith belongs. But, still, there is the nagging matter of actually nailing down a win - a matter that Smith says "is the driving force behind anyone who's out here."

"I want that win. I want it badly. And I'm going to get it one day," he says.

That day could be today. Ben Smith could make golf history this afternoon. But even if he doesn't, he knows he won't have to go out afterward and find a real job. So there's pressure, but not real pressure.