FOR FISCAL 1990, the budget year that ended four weeks ago, the final numbers are now in. The deficit was $220 billion -- the largest since 1986, and almost half again as large as the previous year's. The Reagan-Bush strategy insisted that over time economic growth would automatically close the budget deficits, but the 1990 figures are fatal to that hope. The rise of the deficit helps explain President Bush's turnaround on taxes last spring, and the struggle over the budget that has gripped Washington politics ever since.

It's instructive to look back over the past couple of years to see what went wrong. The 1990 budget was President Reagan's last, sent to Congress just before he left office, and with a few revisions it became President Bush's first. In that version the deficit was calculated as $91 billion, comfortably within the Gramm-Rudman-Hollings Act's limit of $100 billion. By last winter, when the budget was in effect and the GRH limit was no longer binding, the White House revised its estimate upward to $124 billion. Even at that level it would have been on a descending track from the preceding years.

But during the next six months, the estimates of the deficit soared. By last July the White House was publicly using a figure very close to the final one.

There are three major reasons for that gaping difference between the White House's first estimate of $91 billion and the reality of $220 billion. Something approaching $40 billion was Rosy Scenario -- unrealistically optimistic forecasts of economic growth and spending cuts that the administration couldn't get through Congress. That much was pretty widely recognized at the time. But then two things happened that had not been generally foreseen. The economy has slowed down sharply over the past year, costing the federal government nearly $35 billion in tax revenues. At the same time the administration, to its great credit, went to work seriously and vigorously to clean up the S&L industry. It spent $52 billion putting failed S&Ls out of business, an urgent job for which its original budget had made no provision.

The moral of the story is that deficits are very vulnerable both to bad judgment and to unexpected events. A country that runs a low deficit leaves itself with a margin of error that the United States, to its peril, now lacks.