THE NATION'S used-car dealers have spent a great deal of time, effort and campaign contribution money in recent months to convince Congress that they can't live with a proposed Federal Trade Commission rule requiring them to disclose defects in the used cars they sell.

Actually, the rule in question is a very mild attempt to bring a measure of respectability to a market with a reputation for shoddy dealing. When the FTC proposed the rule last August--and rejected the much stiffer controls it had previously entertained--the measure was denounced by consumer advocacy groups as a weak-kneed response to the dealer misrepresentation problems disclosed in six years of investigations and hearings.

The rule does not require dealers to inspect used cars before selling them or make any warranty whatever with respect to their performance. The dealer would simply have to put a sticker on the car listing any defects that he knows about. The FTC estimates the cost of the sticker at 18 cents.

The dealers claim that the possibility of lawsuits by disgruntled customers will force them to inspect all their used cars at considerable additional cost. We can think of a worse place than a world in which used-car dealers made a good-faith effort to let customers know what they're buying. But let's grant that even the strongest rule could be circumvented and that adding costs should not be ignored.

The FTC finds that most car dealers already inspect cars before they buy them. They don't want to get stung. Some, particularly those in the no-warranty-buy-as-is business don't inspect, and they wouldn't have to under the rule. Requiring disclosure of known defects might give a customer an added incentive to sue if he bought a lemon, but the customer would still have to prove that the dealer knew about the defect and disguised it. Without access to dealer inspection records, that would be difficult to prove.

Might it be that some dealers already know a lot they're not telling about the defects of the cars they sell? That thought doesn't seem to concern the more than 205 congressmen who are sponsoring a resolution to overturn the FTC rule. Federal Election Commission records show, by the way, that campaign contributions have gone to about 85 percent of the sponsors from the car dealers' PAC.

Opponents of the rule hope to force the measure from committee, perhaps today, and push it to quick passage on the floor. That would be unfortunate. The rule is not likely to do any harm to honest dealers, and it might do consumers some real good.