ON THE RARE occasions when reporters have been able to question the president about his budget plans for a second term, Mr. Reagan has suggested that they "take a look at the budgets I've already submitted, and look at the cuts that I've asked for and was not given." If Congress had gone along with his ideas, he asserted, the budget deficit would now be $40 to $50 billion lower.

Mr. Reagan, of course, is far too adroit a politician to bore his audience with tedious details. After all, in 1980 he persuaded voters that he had a realistic and, moreover, painless plan for balancing the budget, and no one seemed curious about the fineprint. This time he points to the "thousands of words of documentation" that his budget office has prepared over the last four years as evidence of the solidity of his future plans. But what do those documents reveal?

The Office of Management and Budget says it will provide no list of the alleged $40 or $50 billion in desired savings. But it will offer a few clues. The cuts, it says, include all those requested by the president in March, May and September of 1981 but subsequently rejected by Congress. A search through these requests, however, reveals far less of substance than you might have imagined.

In the first place, Congress gave the president much that he asked for -- including sharp cuts in jobs, services and benefits for low-income people, cutbacks in enforcement of environment and safety programs, curtailment of low-income housing, curbs on health aid for poor families and nursing home residents and so on. As a result the deficit is more than $50 billion lower than it would have been. Congress also did the president the favor of curbing his military budgets. Otherwise this year's deficit would be at least $20 billion higher -- something Mr. Reagan forgets to mention.

Most of the remaining cuts fall in the following categories:

Unthinkable -- the multi-billion dollar May 1981 Social Security cut proposal that the president now all but denies he ever made, for example. Also big cuts proposed for education, foreign food aid and other items now on the president's pet projects list. And extra meannesses targeted for the very poor.

Imaginary -- much of the expected savings were produced by the simple expedient of assuming, in 1981, that programs would cost much less (and federal leases and sales would be more lucrative) in 1984 than could reasonably be expected, even without the high unemployment and rising poverty that occurred. (On the other hand, lower than expected inflation brought the administration some unanticipated windfalls.)

Undoable -- here lie various sensible ideas to curb subsidies for users of waterways, airports and other federal facilities and to limit pork-barrel spending. The savings didn't add up to much, but, given congressional intransigence, the administration hasn't bothered to propose most of them in recent years.

If the president has a plan for narrowing the deficit, it is clearly not to be found in the documents he suggests. The interesting question is whether it is to be found anywhere at all.