The hoopla surrounding the override of President Reagan's misguided veto of a supplemental appropriations bill is a classic case of molehill-mountain exaggeration. It hardly marks a reversal of the long-term direction in which Reagan is pushing policy.

To see the larger trend, read "The Reagan Experiment," the just-published volume by a group of scholars attached to the Urban Institute. What they demonstrate is that in his first year in office, Reagan rolled the tax and spending policies of the federal government backward a decade.

The Urban Institute volume is the first installment in a continuing foundation-financed effort to assess the impact of Reaganism. It has its limitations and its biases, as any such work must, but it is far and away the most thoroughly documented such analysis to come on the scene.

The editors -- John L. Palmer and Isabel V. Sawhill -- are agnostic on the proposition the Republicans are advancing in this mid-term campaign: that the Reagan program "deserves more time" to produce the advertised results of economic growth and more jobs. They merely observe, as anyone would, that it has not produced those results as yet.

But the volume does offer clear and unequivocal supporting evidence for the Democratic Party's counterclaim that the Reagan program is "unfair" in its treatment of individuals and regions.

The data demonstrate that the "working poor" -- those just above the poverty line -- are being hurt by Reagan's tax and budget policies. They must wait until 1985 for any significant tax relief; meantime, they are the ones who are being weeded out as the social programs are targeted to "the truly needy."

More broadly, the studies show that through 1984, at least, the Reagan policies mean no benefit to most families below the $15,000 level, only modest benefits to those between $15,000 and $50,000, and substantial advantages only for those above the latter figure.

Similarly, the reductions in domestic expenditures and the increase in defense budgets "will aggravate the imbalance in regional growth," the report shows. The winners in Reagan's game are the Sun Belt and energy-rich states. The losers are concentrated in the Midwest, Appalachia and much of the Northeast.

These are facts. Whether one thinks they are good or bad social policy is a question, as the authors point out, that is subject to legitimate political debate. But after reading this book, no one can think the debate inconsequential. Nor can one take an offhand attitude toward the mid-term election.

I was intrigued by the "calendar" the authors created to measure the impact of the changes Reagan achieved in his first whack at taxes and spending. Those changes, if carried through, would by 1985 reduce federal domestic spending as a share of the gross national product to about the level of 1974-75. They would reduce the tax burden, as a share of GNP, to the level of the early 1970s. In some areas of domestic spending, like employment and training programs, Reagan has already rolled the clock back to a pre-Great Society period.

In his grand design for federalism, Reagan is proposing to "restore economic policy and intergovernmental relations to their status before the New Deal," the authors observe.

Reading this, an antic notion crossed my mind. If Reagan were successful in repealing almost a decade of previous domestic policy each year of his presidency, and if he were given two full terms to operate, by the time he left the White House he would have moved things back just about to the time of his own boyhood in the William Howard Taft administration.

And since, like all of us, he thinks of his childhood as a golden era of life, that would probably be what he would consider a most satisfying achievement.