The budget President Reagan sent to Congress last week calls for a deficit of $130 billion in fiscal 1989, theoretically within a revised Gramm-Rudman-Hollings target. That's down $17 billion from the $147 billion now estimated for this year, fiscal 1988. But almost everybody else, including the nonpartisan Congressional Budget Office, thinks the real deficit number is more likely to be $160 billion to $170 billion -- and if there's a recession this year, it could be nudging the $200 billion mark again.

Even if there is no such calamity, it's worth remembering that the Reagan administration last year at this time forecast a deficit for fiscal 1988 of only $108 billion -- $39 billion under the current estimate.

When this was called to the attention of a White House staff member, the response was: ''I just couldn't believe we had it {the deficit estimate} that low last year.''

The main difference between the president and the CBO on the fiscal 1989 estimates is in underlying economic assumptions. Beryl W. Sprinkel, chairman of the Council of Economic Advisers, put the economic growth rate at 2.4 percent for calendar 1988 and at 3.5 percent for calendar 1989. The CBO and others think that may be too optimistic for next year.

Moreover, the administration forecast calls for lower interest rates this year and next, while the CBO sees substantially higher rates -- both short term and long -- in 1988 and 1989. Obviously, if the CBO comes closer in its growth, inflation and interest rate forecasts, the deficit will be higher than the Reagan administration is saying.

But all budgets must include some economic guesswork, and it's hard to fault the administration for putting the best possible face on things. There are other parts of this budget, however, that are less defensible. For example, the administration counts on one-time sales of government assets and loans of $10 billion. But the cosmetic deficit-reducing benefits of some of those sales will involve a real cost in terms of income later on.

Then, there are some eyebrow-lifting assumptions of ''savings.'' To take what appears to be an egregious example, the nation's thrift institutions and banks are still in real trouble. But, according to budget boss James Miller, the administration is claiming a $2 billion to $4 billion budget reduction ''from a reduced need to bail out bankrupt banks and S&Ls.''

The budget document doesn't deal with the full S&L story. No mention is made of prospective $7 billion to $10 billion bailouts for Texas S&Ls alone for the next next two years; conveniently, Federal Home Loan Bank Board plans for this probability were not completed by the time the budget went to press. But Congress will eventually have to provide for that contingency.

Moreover, the budget claims that for fiscal 1989, Federal Deposit Insurance Corp. bailouts for defunct banks will cost only $1.5 billion, against $3.8 billion in fiscal 1988. However, a check with the FDIC indicates the agency expects that, once again, there will be a repetition in 1988 of the record 200 bank failures in calendar 1987.

Where, then, is the ''reduced need'' asserted by Miller, and how can the budget office defend its claim of a $2.3 billion savings in the FDIC budget? The answer, according to FDIC Chairman William Seidman: the assumption is made that there will be dramatically fewer commercial bank failures in the first half of calendar 1989. There is no available evidence to support that claim. And although Seidman won't take issue with such a hopeful projection, he admits: ''That {assumed saving} could be wiped out with one big one {an unexpected bank closing}.''

But whether fiscal 1989's deficit turns out to be Reagan's $130 billion, the CBO's $160 billion or some other higher or lower number, the real story of Reagan's final effort is that it symbolizes what surely must be one of the most negative results of his presidency.

Against the promise made during the first Reagan campaign of a balanced budget in three years, we have had a record series of eight huge, consecutive deficits -- three of which exceeded $200 billion each -- that added $1.3 trillion to the national debt, or well over the total amount accumulated by all of Reagan's 39 predecessors.

In the end, much to the consternation of the GOP right wing, Reagan's commitment of eight years ago to ''get government off your backs'' has not been fulfilled. Generous entitlement benefits for middle-income America remain in place, although many programs for the really poor took disproportionate cuts.

Reagan's economic legacy thus poses a massive problem for the next president and future Congresses. So far, the current crop of Democratic and Republican candidates for office has turned its back on the only available ways of dealing with the deficit problem -- raising taxes and reducing middle-income entitlements. Eventually, one of the candidates will have to put campaign platitudes aside and deal with the real problems bequeathed by Ronald Reagan.