IF POLITICAL conservatives are troubled about the Postal Service's plan to honor three renowned liberals with stamps next year, then they surely will want to rally around a new 40-cent stamp to be released Thursday in Monroe, La.

It honors a colorful, storied conservative whose exploits against the Chinese Communists during World War II and afterward have made him one of the heroes of the political right.

Lt. Gen. Claire Lee Chennault, the blunt-spoken aviator who in the 1940s created the "Flying Tigers," may not be as well known today as Hubert H. Humphrey, Chief Justice Earl Warren and New Mexico's Sen. Dennis Chavez -- the three liberals who will get their stamps next year. But Chennault also was a giant in his day. A square-jawed, outspoken man, he made a career of defying tradition and proving his critics wrong.

After increasing deafness forced him to retire from the Army in 1937, Chennault went to China where he was hired to rebuild the Chinese Air Force to meet the expected Japanese invasion. The attack began a few weeks later and it quickly was followed by pressure on the U.S. government to sell 100 P-40 fighters to China.

A group of American volunteer flyers assembled by Chennault and known as the Flying Tigers began flying sorties in those planes 10 days after Pearl Harbor. The group was absorbed in 1942 by the 14th Air Force but its fame and that of Chennault spread around the world.

After the war, Chennault became a strong supporter of the Chinese Nationalists and repeatedly urged the United States to help the government of his close friends Generalissimo and Madame Chiang Kai-shek fight the Communists on the mainland. The general adopted Taiwan as his home, divorced his wife of 35 years and married Anna Chan, a 24-year-old newspaper reporter there. He established the Civil Air Transport, the international airline he operated until his death in 1958.

Anna Chennault, still a major figure in Washington society, will join Regional Postmaster William A. Campbell Thursday to dedicate the stamp in ceremonies at the Northeast Louisiana University in Monroe. The blue stamp was designed by Chris Calle of Stamford, Conn., based on a wartime photograph of the general.

It was printed by the Bureau of Engraving and Printing using the intaglio process and with the engravings of Thomas Hipschen and Dennis Brown. The stamp will be part of the Great Americans series that has been used as part of the regular stamp series sold at all post offices since 1980. Taiwan also will honor Chennault this year with a stamp.

Humphrey, a Democrat and the 38th vice president, Warren, a Republican and the 13th chief justice, and Chavez, a New Deal Democrat and champion of various Hispanic causes, also are to be honored in the series next year. Their selection comes as the Postal Service seems to be showing a waning interest in the series.

Earlier this year it announced the high-valued stamps in the series will be gradually replaced by commemorative size stamps that feature wildlife paintings. In addition, the service has approved, but not announced, a new first-class stamp that will carry a drawing of a doe. All that is needed for that stamp to go into production is for the Postal Rate Commission to fix the next rate for first-class mail.

As expected, the service will continue its practice of having a supply of stamps featuring the flag at all Post Offices. One of those new 1991 flag stamps will honor the 50-year-old Mt. Rushmore National Memorial in South Dakota and another will mark the 125th anniversary of Memorial Day in Waterloo, N.Y.

Among the commemoratives to be released next year will be a long-awaited booklet of stamps drawn by caricaturist Al Hirschfeld saluting famous American comedians. The booklet began as a single stamp to honor Stan Laurel and Oliver Hardy, but the service has not named the others who will be included.

Coin collecting will get a commemorative, as will lyricist Cole Porter, playwright William Saroyan and inventor Jan E. Matzeliger. Matzeliger will join the Black Heritage series for his creation of the lasting machine, which revolutionized the shoe industry. Vermont and the District of Columbia will get stamps for the anniversaries of their founding.

Basketball's centennial will be marked and the service will continue its promotion of the Olympic games with another series of sports stamps. Perhaps the highlight of the year will be a set of 50 different wildflower stamps, similiar to the sheet of wild animal stamps that the service issued in 1987 to much acclaim.

COLLECTORS wishing to secure first-day cancellations of the Chennault stamp may either prepare their own envelopes with stamps purchased after Sept. 7 at their local post office or request postal workers to affix the stamps on up to 50 envelopes at a price of 40 cents per stamp. Collectors who purchase their stamps should mail their envelopes to: Customer-Affixed Envelopes, Chennault Stamps, Postmaster, 501 Sterlington Rd., Monroe, LA 71203-9991. Requests for postal workers to affix the stamps should go to: Chennault Stamps, Postmaster, 501 Sterlington Rd., Monroe, LA 71203-9992. All requests should be postmarked by Oct. 6.

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