ON THE TAX bill, the White House talks hotly about its great struggle with its powerful and dangerous enemy entrenched in Congress. That's pure theater. The Reagan tax bill is now rolling through Congress with a speed and success that is becoming dangerous to the administration itself. The president is getting even more than he asked, and the federal tax system is being changed more profoundly than he, or his Treasury secretary, seem to realize.

Mr. Reagan and his friends, a combative and energetic crowd, came to Washington full of zest for battle. They were eager to batter and force the gates of the status quo. They had been told that those gates were double-barred against them, and the adrenelin was running high. They bagan to pound and push, vigorously. And then they realized that they were pushing on an empty door.

The Democrats have abandoned their opposition on the central issues of the tax bill. A political tradition has, at least temporarily, collapsed. It was Wall Street, not the congressional Democrats, who forced Mr. Reagan to scale down the first phase of the cut -- and that's the only concession that he's had to make.

Much of the supply-side theory was a labyrinthine rationale for reducing the top income rate from the present 70 percent to 50 percent over three years. Congress -- both parties -- shrugged and said, "Why wait three years?" It will drop to 50 percent in the first year.

That would automatically reduce the top rate on capital gains to 20 percent, an exceedingly modest burden. The nays? Hearing none, so ordered.

The Democrats on the Ways and Means Committee have produced a better depreciation formula than the president's. But since it doesn't produce as large tax cut for business, the Democrats have added to their alternative a sufficiently huge reduction in the corporation income tax rate to match the president's offer. As a practical matter, either of these proposals will come rather close to abolishing the corporation income tax.

In addition -- this one wasn't the administration's idea -- Congress is now moving toward a very large reduction in the inheritance tax. The Democrats say that it will help the family-owned farm and small business. No doubt it will help them -- among others.

These four broad changes in the basic tax structure have one characteristic in common. They all concentrate benefits on taxpayers with unusually high incomes. The tax bill is not yet in its final form, and its precise impact cannot be estimated. But it appears tha tthe principal beneficiaries are going to be people with incomes upward of $50,000 -- the top 5 percent of all taxpayers.

While the Reagan administration meant to cut taxes for the wealthy, it has overshot its mark without, apparently, fully realizing it. The administration also intended to lighten the load for the middle classes, but for them the bill will merely offset in tax increases that which inflation imposes.

There's no evidence that anyone of either party, in the administration or Congress, has given much attention to the way in which these coming tax cuts play against each other. The corporate tax cut, for example, will raise the prices of stock whose owners can sell at the lowered capital gains rates. Fast depreciation invites a whole new generation of tax shelters. The people using these shelters will not, generally speaking, be the truly needy.

Senator Dole, the chairman of the Finance Committee, tried to deflect debate on that point the other day with the exasperated comment that "this is a tax program, not a redistribution program." Wrong: It's both. The difference is that the tax cut is intentional, but the income redistribution is uncalculated and heedless.

Mr. Reagan wanted to give a little help to wealthy investors, who said they were overtaxes. He now risks succeeding to a degree that becomes an embarrassment and a liability, to both him and the hapless Democrats.