IT NEVER FAILS: whenever Washington is trying to show Congress the door during one of those awful adjournment scrambles, you have to keep a sharp eye on the silverware. While pay raises and perks held center stage, legislators found yet another little way to keep themselves some small change with a small act against the District of Columbia. In the name of congressional sovereignty, the Constitution and nonprofit protectionism, the members have approved a measure creating a duty-free PX for themselves and anybody else who wants to evade the D.C. sales tax that other stores in town have to levy.

At issue is a tiny souvenir shop run by the nonprofit U.S. Capitol Historical Society, a fine organization that restores paintings in the Capitol, produces historical calendars that members hand out to constituents and publishes educational material for visitors and students. Stop right here, because if that were all the shop did, an exemption from D.C. sale taxes could indeed be justified as a federal/congressional prerogative.

But this shop sells general items such as cameras and film, which city officials argue constitutes unfair competition with local merchants. If the measure is signed into law by President Reagan, it also could clear the way for other organizations that provide services for Congress -- office equipment sales, for example -- to claim tax exemptions, too.

Before you know it, the Great Rotunda will be converted into a supermarket and the Rayburn Garage will become a used-car lot.

The District claims that it is owed $740,000 in back taxes for all the noneducational items sold by the shop, and has been in court on the matter since 1979. But why should it come down to courts and special laws, when reasonable people could agree to limit tax-free sales to Historical Society materials and other noncompetitive congressionalia. If Mr. Reagan has any respect for the integrity of local home rule in the District, not to mention free enterprise and fair competition, he should veto or ignore this mean little measure.