THE WHITE HOUSE has chosen a theme song for the budget this election year. It is a lullaby, and you heard the first few trial verses this week. The early formulation goes about like this: maybe it's true that the deficit problem hasn't been solved forever, but certainly it has been solved for the rest of Ronald Reagan's term, so don't bug us. "Basically the summit team dealt with all the issues," presidential spokesman Marlin Fitzwater told reporters as the president began work on the spending plan. "The message to markets is . . . we've got the deficit on a downward trend."

And indeed, when the president's budget is submitted in February, it will describe such a trend. You can bank on it. The deficit was about $220 billion in fiscal 1986 and about $150 billion last year. The budget will show it -- thanks to the combination of tax increases and spending cuts agreed to at the fiscal summit last November -- holding at about $150 billion this year, then declining further, to precisely the Gramm-Rudman target of $136 billion in fiscal 1989. By then, of course, this administration's stewardship will be over, and if things don't work out as well as forecast -- well, that will be someone else's fault and problem.

But these budget estimates will not be realistic. On the contrary, congressional and outside experts believe that -- in the real world as opposed to the confections of the budget process -- the deficit this fiscal year is likely to be up a little, to perhaps $160 billion. And unless the president and Congress cut it further, they think it will also be about $160 billion in fiscal 1989.

The students of the process will tell you, first, that the 1987 figure was artificially low. About half the large decline from 1986 was genuine -- defense and domestic spending both were curtailed -- but about half was one-year-only money from either tax reform, which caused a rush in capital gains, or fake accounting. The "real" or underlying 1987 deficit was thus about $190 billion.

For the current year the deficit is likely to come down further by virtue of the summit agreement, but more likely to the neighborhood of $160 billion than the $150 billion that was promised. For one thing, the economy won't perform as favorably as was assumed for summit purposes. For another, not all of the summit cuts were genuine or realistic, either; there will be some slippage. And in 1989 the experts think there will be more slippage in both the agreement and the economy.

The Congressional Budget Office is likely to reflect at least part of this more pessimistic outlook in the forecasts it will make once the president's budget is submitted. Congress will then be confronted with the same problem it faced last year. The administration will be able to say, on the strength of its rosier assumptions, that it hit the Gramm-Rudman goal while not proposing many new cuts in an election year. Lots of gain and little pain will be its promise. Congress will be forced either 1) to go along with the puffy estimates or 2) to accept the grimmer prospect laid out by its own staff, in which case it will have to either cut more -- perhaps as much as $20 billion -- or accept the blame for a higher deficit.

That's how the playacting will go. The reality beneath is that the deficit problem has not been dealt with, merely shifted to another time and the next administration. Remember that when next you hear that lullaby.