PRESIDENT REAGAN is moving fast on his tax bill. Skillful legislative tacticians that they are, he and his staff are exploiting the momentum of their budget successes. When he brought out his slightly revised version of the tax bill last week, it was a declaration that he will spend no more time in negotiation with the House Democrats. The president is now forcing the pace in Congress, choosing to risk a limited defeat in the House rather than taking up complex and time-consuming changes with the Democrats.

The schedule is dominating the substance of the tax bill. To keep it moving along rapidly, the administration has adamantly resisted any elaboration of its original simplicity -- not to say crudity. This bill reflects the president's judgment that a series of broad cuts in the personal income tax rates is now more urgently important than anything else in tax policy. The result is going to be a tax cut bigger than it ought to be, less fair than it ought to be and less effective than it could have been.

The revisions in the bill that Mr. Reagan announced Thursday were addressed to the fiscal conservatives -- not only Democrats in the House but, much more important, Republicans in the Senate. Some of those senators have already let him know that they are deeply concerned by this tax cut's implications for the federal deficit. In response, Mr. Reagan now proposes for the first year a 5 percent rate reduction beginning in October, instead of 10 percent in July. But the two subsequent annual cuts remain at 10 percent each. He has changed the depreciation provisions for business, limiting the enormous revenue losses that the original bill would have allowed over the years. But even with those refinements, his promises of a steeply declining deficit remain implausible.

As the bill moves through the House, the Democratic majority will probably 1) try to shorten it to two annual cuts instead of three, with 2) a smaller total reduction that 3) provides more for taxpayers in the low- and middle-income ranges. On all three points the Democrates will be right. But if the president loses on any of them, he can hope to recoup in the Senate -- as long as he manages to persuade all of the Republicans that they are not merely voting for another big budget deficit in 1984.

As the Reagan tax bill now stands, it means -- at even a moderate rate of inflation -- higher income taxes by 1984 for low-income families. Because of inflation, it means little or no real tax cut for any but the very properous. It would abolish the extremely high rates on large incomes from dividends and interest; that's reasonable enough, but simultaneously it ought to abolish some of the more questionable shelters and avoidance schemes. Instead, it ignores them. The provisions for business taxes remain recklessly indiscriminate. They still offer a bonanza to the oil refiners, who are not currently among the truly needy, while doing astonishingly little for industries like the automobile manufacturers, who are desperate for capital.

If Congress obediently keeps to the president's tight schedule and rushes to enact the tax bill this summer, it will not have done its job. Congress' responsibility is to slow the process down, provide time for careful consideration and refine the legislation that the president has drafted.