When Bill Bradley was the scoring sensation of college basketball 20 years ago, he once tried to explain how he shot the ball unerringly without seeming to look at the basket. You needed "a sense of where you are," he told John McPhee, and McPhee seized upon the phrase as title and theme for a New Yorker magazine portrait that told as much about the man as the player.

Bradley has crossed the Hudson from one elite institution (the New York Knicks) to another (the U.S. Senate, as New Jersey's Democratic senior senator). But he still radiates that sense of knowing where he is -- especially on the hot topic of "tax simplification."

Bradley has been an apostle of the cause for 10 years, offended (or for so cool a customer, a better word is annoyed) by the inequities, oddities and inefficiencies of the Internal Revenue Code. As Bradley notes, it's longer now than the Bible, and more people are paid to cope with its intricacies than to teach high school English.

Bradley, the son of a Missouri banker, was known in his pro-basketball days as "Dollar Bill." He is unlikely to be accused of embracing tax reform in envy of those whom the tax code has treated well.

But his sense of the absurdity of income taxation, American style, was first aroused when a tax attorney told him (he was 23) "that I had to make a decision about how much I wanted to pay in taxes." He could, the taxman explained, take his pay as salary, current or deferred, in whole or in part, or as property, or as a consulting contract, or as insurance and pension benefits, or as payments to his own corporation.

Bradley was flabbergasted; this was puzzling stuff at 23. He apparently has been pondering the matter ever since -- that was in 1967 -- with a growing conviction that the tax code cries out for changing. His own objections center not only on its unfairness, but on the incentives it offers upper- bracket taxpayers to take socially unproductive investment plunges to avoid taxes. The "fair tax" being pushed by Bradley and Rep. Richard Gephardt would, like the Reagan-Treasury draft, reduce the 12-rate system to three basic (and lower) rates, while stripping away many encrusted deductions, preferences and credits.

The Bradley-Gephardt bill does depart in several significant respects from what is now known of the administration plan: it would, for instance, maintain the deductibility of state and local taxes. Probably, no tax simplification that ignores the restlessness over high local property taxes implicit in Proposition 13 and other tax rebellions is likely to fly, especially if it caters in other ways to special interests.

Perhaps the major question about tax simplification is whether a fragmented Congress can muster the procedural discipline, and bipartisan consensus, to pass a "clean bill" -- i.e., one that doesn't become a hatrack for everybody's pet tax-avoidance scheme. Bradley wants to help, but he's beginning to fear that by the time the administration plan emerges from this prolonged nibbling it may be fatally compromised by "special interest" concessions.

Even for people of relatively low partisan temperature such as Bradley, tax simplification as a "party realignment" plan is likely to be a bit indigestible. He and Gephardt (and other Democrats) have been on record on this issue much longer than the Reagan administration.

The prolonged delay in the unveiling of the administration proposal seems designed to coincide with Congress' Memorial Day recess, so that the president can have the stage to himself May 28 when he makes his plan public.

That's okay with Bradley, though he recalled that as of several weeks ago the president didn't know his own plan would raise corporate taxes. If he's going to solo, Bradley says with a twinkle, "he'd better know what's in his own bill." Bradley, you may be sure, knows what's in his. It's a part of knowing where you are.