IF ANY MEMBER of Congress doubts that the Soviet Union is pressing its campaign for the Senate to ratify the treaty eliminating short and medium range nuclear missiles, then they need to check their mail.

If they've received a recent letter from someone in the USSR, chances are it may bear the latest proof of the Soviets' public relations campaign -- a stamp that salutes the treaty.

The stamp was issued December 17, nine days after Soviet leader Mikhail Gorbachev and President Reagan signed the treaty in a White House ceremony. But the stamp doesn't show the White House.

It features the Capitol (described in a Soviet press release as "the U.S. Congress Building"), the Kremlin, the U.S. and Soviet flags and a globe crossed by an olive branch. The Russian text, perhaps reflecting the tendency of Soviet leaders to be long-winded, says "The Intermediate Range Nuclear Forces Treaty is a First Step on the Road to Nuclear Free World, December, 1987."

It's not the first time the Soviets have issued a postage stamp with an American theme, but such stamps are "rather exceptional," according to James A. Helzer of the USSR Stamp Agency in North America.

In 1953, the Soviets issued a stamp honoring Benjamin Franklin, and in 1975 the joint U.S.-Soviet Apollo-Soyuz venture brought stamps from each country. In 1985, the USSR issued a stamp for Samantha Smith, the American schoolgirl who died in an airplane crash after visiting the Soviet Union, and last year it released a stamp honoring John Reed, the American who wrote the celebrated "Ten Days That Shook the World," about the Russian Revolution.

Helzer said the latest stamp was "obviously planned well in advance" of the Washington summit, although announcement of the stamp was not made in the United States until this month. The stamp was printed by the gravure process which takes longer than the offset methods many countries use to quickly turn out timely issues.

The Soviets long have issued stamps to support various propaganda themes, such as their support for various world conferences of socialist countries. Last year it issued a set of three stamps and a special souvenir sheet commemorating a joint Soviet-Syrian space flight as well as pairs of stamps to celebrate friendship with the government of Mozambique. The stamps typically carry the flags of the two nations and have figures in the heroic style typical of much contemporary Soviet art.

The Soviets don't announce how many stamps they print, and Helzer, whose Cheyenne, Wyoming firm sells stamps to collectors and dealers at face value, says it's too soon to measure any surge in demand for the nuclear weapons treaty stamp.

Readers of Linn's Stamp News, the most widely distributed stamp publication in the United States, have had their say about the stamps of 1987 and, as for the past 16 years, they have agreed that the U.S. Postal Service is at its best when it issues sets of stamps on the same subject.

Last year's colorful sheet of 50 wildlife stamps was the hands-down winner as the most popular stamp of the year in a reader survey that drew nearly 3,500 ballots. The wildlife stamps also were voted the best designed commemoratives, followed by a booklet of five historic locomotives stamps.

Among the regular, or non-commemorative, stamps the winning design was the 22-cent flag with fireworks stamp. It was also voted the most important regular issue by an overwhelming majority.

The day-glow red commemorative celebrating the Pan American Games in Indianapolis was voted the worst-designed commemorative of 1987, and the signing of the Constitution stamp was voted the most important commemorative.

A booklet of 10 special-occasion stamps (i.e. Get Well, Happy Birthday) was voted the least important commemorative by the readers, followed by the stamp honoring Certified Public Accountants, which was ordered printed by a postmaster general, not by the citizens advisory committee that is charged with choosing stamp designs. The worst-looking regular stamp was a 14-cent stamp showing a pensive Julia Ward Howe, and a $5 stamp honoring writer Bret Harte was voted the least necessary.

Bill McAllister is a member of The Washington Post national staff.