Ten years after Richard Nixon made his historic first trip to China, another conservative American President has moved significantly to improve relations between the United States and the world's most populous country.
In agreeing to reduce arms sales to Taiwan, Ronald Reagan has incurred the censure of congressional conservatives, some of whom suggest that his controversial tax increase bill may be imperiled by what seems to be yet another shift away from his preelection positions.
But the President and State Department officials insist that the United States-China joint communique, issued Aug. 17, does not reduce the administration's concern for ''the well-being of the people of Taiwan.'' They emphasize that military support for Taiwan will continue sufficient to meet the island's defense needs, and that they will ''keep the situation under review.'' They repeatedly point to China's declaration in the communique that its ''fundamental policy'' is the peaceful resolution of Taiwan's future.
''The situation in the Taiwan Strait has been more stable in the last year or so than at any preceding time,'' a senior State Department official told reporters in a background briefing. ''The tensions have been at an all-time low.''
At the same time, however, relations between the US and the People's Republic of China -- at least on this crucial question of Taiwan -- have been anything but harmonious in recent months. China has been insisting that the US stop aiding Taiwan militarily, and a top US diplomat recalls a period earlier this year ''when the situation seemed tense indeed.''
Those who oppose the most recent US-China communique see this as part of China's recent effort to position itself more evenly between the US and the Soviet Union and to reassert its dominance in the third world.
''I think the Taiwan issue is being used somewhat as a lever to get the United States to make concessions on a number of points,'' says Robert Downen, East Asian expert at Georgetown University's Center for Strategic and International Studies.
Dr. Downen calls the new communique a ''severe disappointment'' that ''appears to violate the intent and some provisions of the Taiwan Relations Act'' (which he helped draft in 1979 as an aide to Republican Sen. Bob Dole of Kansas).
The House Foreign Affairs and Senate Foreign Relations Committees have oversight over the Taiwan Relations Act, but conservatives such as Sen. Barry Goldwater (R) of Arizona acknowledge that there is not much they can do regarding this latest diplomatic initiative between the US and China.
Aside from the question of military sales to Taiwan, relations between the US and China have been steadily improving. The two countries are expected to reach a new textile agreement this year, according to US negotiator Peter Murphy, who currently is in China. China is this country's fourth largest textile supplier.
The US and China reportedly are discussing a possible agreement on peaceful nuclear cooperation enabling American firms to help develop Chinese nuclear power. The US has eased regulations for licensing exports for other construction projects.
At the same time, however, President Reagan this week emphasized his ''long-standing personal friendship and deep concern'' for the Taiwanese and his commitment to ''maintaining the full range of contacts'' between the US and Taiwan.
As expected, Taiwan responded unfavorably to the communique, the Foreign Ministry saying China is ''trying to pave the way for their military invasion of this country.''
Under the communique, the US agrees ''that its arms sales to Taiwan will not exceed, either in qualitative or in quantitative terms, the level of those supplied in recent years . . . and that it intends to reduce gradually its sales . . . leading over a period of time to a final resolution.'' Under this agreement, the US and Taiwan will continue to co-produce the F-5E fighter.