The election of Ronald Reagan began a period of intense testing by China and Taiwan of the president's political constancy. The People's Republic, which had earlier stood by and merely grumped at the continued American arming of Taiwan, started demanding that Mr. Reagan cut Taiwan off, or at least cut it back. Not to be outdone, the authorities on Taiwan pressed their arms demands all the harder.

Communist Peking obviously has hoped to test the new president's anti-Sovietism. Its implicit threat is to heal some part of its rift with Moscow if the United States will not act satisfactorily on Taiwan. Non-communist Taipei's test is of Mr. Reagan's anti-communism. Its threat, not so implicit, is to embarrass him in the eyes of the important part of his core constituency sharing his fierce attachment to Taiwan.

Over the months of pulling and hauling, the contest has come to focus on a demand by Peking that the United States agree to halt, on terms to be established, arms sales to Taiwan. Peking views the sale of arms to a territory it regards as a province as unacceptable interference in its internal affairs. The State Department, under former secretary Alexander Haig, had suggested to the president a formula linking the provision of arms to the expected continued progress toward a peaceful settlement between Peking and Taipei: the more progress, the smaller the need for arms. That was a happy formula, permitting Mr. Reagan to tread a fine but fair line between respecting the nationalistic imperatives of a strategic partner and keeping faith with an old friend.

Late last week, however, a group of Mr. Reagan's most conservative domestic supporters warned him not to take that course. Their obvious purpose was to gain a commitment from Mr. Reagan during the period when there is no strong voice at the State Department to defend a sensible diplomatic perspective.

How bizarre that Ronald Reagan should be told not to "abandon" Taiwan. Mr. Reagan is the last man in America to need instruction on this issue from conservative quarters. For him to let himself be influenced by people divorced from current geopolitics and bewitched by rusty symbolism would represent precisely the caving in to pressure that the new China lobby says it cannot abide.