Rep. Richard Gephardt has made another impassioned attempt to revive his moribund amendment to the omnibus trade bill {op-ed, Jan. 25}. Unfortunately, his amendment is based on two questionable premises: (1) "unfair" trade practices have caused the trade deficit, and (2) threats to bar imports can open markets to U.S. exports.

Regarding the first, the International Trade Commission -- hardly a center for dogmatic free-traders -- estimates that only 10 to 12 percent of the trade deficit can be blamed on other countries' "unfair" trade practices. This estimate is not so surprising. Can anyone really believe that the swing from a $12.5 billion merchandise surplus in 1980 to a $38.2 billion trade deficit in 1983 occurred because other nations suddenly became vastly more protectionist?

Even if such policies really were responsible for the trade deficit, punishing American consumers and exporters through protectionism will not induce other countries to change their trade policies. It is foolish indeed to propose closing U.S. markets to Japan, Korea, Taiwan and West Germany -- countries that bought more than 32 percent of U.S. agricultural exports in 1986.

The truly effective way to prod other countries to lower their trade barriers is to offer to reduce ours as well, not to threaten to erect new ones. That is the real lesson to be drawn from the U.S.-Canadian free trade agreement that Rep. Gephardt wholeheartedly applauds.

MARY K. ALEXANDER

Director of Trade Policy, Citizens for a Sound Economy

Washington