THE TREASURY'S tax reform proposal is both constructive and courageous. It is also highly unlikely to be enacted. The unseemly speed with which President Reagan is taking his distance from it is not a good omen. This project springs from his own line in the State of the Union Message last winter, in which he promised "a plan for action to simplify the entire tax code so all taxpayers, big and small, are treated more fairly." Now that he's got it, the whole project seems to have lost its appeal. The months before the election didn't seem to the White House like a good time to take it up. Evidently the months after the election are not such a good time either.

The Treasury has outlined a semi-flat tax, with a top rate on personal income of 35 percent rather than the present 50 percent. Taken all together, this plan would shift the burden of federal taxation in a small but welcome degree back from individuals to corporations. At the same time, it would eliminate some of the more gross inequities among different industries. Secretary of the Treasury Donald T. Regan pointed out yesterday that, to take one conspicuous example, banks pay almost no income taxes. The oil industry's cherished preferences would go. The corporate income tax rate would come down, but the most favored and heavily lobbied industries would have to pay taxes more nearly equal to those of other businesses, such as those in the high technologies.

It is correct to say, as a succession of presidents have said, that the present tax system is abominably complicated and basically unfair, taxing people with similar incomes at very different rates. It is correct to say that, in corporate taxation, the discriminatory effect among different kinds of business is outrageous. "We don't feel we should have an industrial policy that's disguised as tax policy," Secretary Regan said -- and the wrong industrial policy at that. It is reasonable to believe, with Mr. Regan, that tax simplification can diminish the vast diversion of brains and money into the field of tax avoidance and release resources that will make the economy grow faster.

But repealing all those special preferences and exemptions is not a new idea. Some of them have histories that go back through decades of successfully repelling the reformers' attacks. Revision of the tax code, on the drastic scale envisioned by the Treasury, would have a chance of success only if it were forced forward by a vigorous and determined president. There's only one candidate for that job, and he doesn't seem to be interested.

Without his active support, the tax reform project will degenerate into a futile debate that only diverts attention from the budget and the need to change its direction sharply. The chief danger continues to be paralysis at the White House on taxation, on the budget, and on everything affecting economic policy.