BACK WHEN the District of Columbia campaign finance law was passed in 1974, most of us assumed that one of the things it should try to prevent was the concealment of large contributions to candidates. So the law very carefully required the disclosure not only of actual contributions but also of pledges and other agreements to make contributions.

Now it seems there is another problem: some politicians may want to inflate rather than hide the contributions they have received. Here is Dr. Morris Harper, a candidate for the Democratic nomination for mayor. Dr. Harper is a physician of good reputation who has some intelligent things to say about city government. But he has never been involved in politics before, and few took his candidacy seriously until he announced recently that he had raised $131,500 for his campaign since January.

Or had he? Post reporter Eric Pianin got in touch with a number of people listed on Dr. Harper's disclosure form as having contributed between $1,000 and $2,000 and found some who said they had contributed or pledged much smaller sums or nothing at all. Spokesmen for the Harper campaign acknowledged that only $56,000 in cash had been received, and said the balance represented pledges.

Whether Dr. Harper or his campaign violated the D.C. campaign finance law by making an inaccurate report is being investigated by the D.C. Office of Campaign Finance. Whatever the outcome, the episode suggests that it may be in a candidate's interest now to overstate the amount of money raised, and that the requirement that pledges of all kinds, "whether or not legally enforceable," be reported gives candidates a basis for such overstatements. After all, an ingenious candidate can solicit and receive verbal pledges of vast support that any serious politician would know were uncollectble.

The federal campaign finance law was revised in 1979 so that it no longer requires the reporting of pledges and of promises to contribute. That is a change the District should consider when it gets around to campaign finance laws after the election. Alternatively, it should at least require candidates to distinguish in their pledges between actual contributions and mere pledges; and that ownly written pledges be required to be reported. The strange realities of politics can make even the most carefully drafted of laws obsolete overnight.