NOW that the 104th United States Congress has reconvened, it is up to the Senate to save US support to the United Nations Population Fund (UNFPA). The House of Representatives voted last May to reinstate the Reagan-Bush policy of withholding donations to UNFPA, the largest multilateral source of population assistance and family-planning services to the world's poorest countries. The recipient countries are home to over 90 percent of the nearly 100 million people added to the world each year. Since the mid-1980s, opponents of family planning, foreign assistance, and China in general have accused the People's Republic of instituting forced abortion in order to uphold its policy of one-child per family. As a contributor to the Chinese national population program, UNFPA was deemed by Presidents Reagan and Bush to be unworthy of receiving US funds. Meanwhile, Bush approved the sharing of nuclear technology with China, asking in return only that China promise not to use that technology for weapons. The policy remained intact for some seven years despite the following facts: (1) The UN Population Fund was never found to have funded abortions, coercive or otherwise. Its support went into programs ranging from the Chinese census to producing an intrauterine device safer than the domestic product; (2) Proponents of UNFPA wrote into legislation to overturn the Reagan-Bush policy a segregated-funding clause to assure that US donations to the fund could not be used in China; (3) Congress consistently voted to restore funding for UNFPA, only to be thwarted by a Bush veto of the foreign-assistance bill and several subsequent veto threats; (4) Not a single UNFPA donor country chose to follow the US example, and all continued to contribute to the agency. When Bill Clinton was elected president in 1992, he wasted little time in announcing the resumption of funding for UNFPA, and Congress backed him up. Only last year Congress approved $650 million for overseas population assistance, including $40 million for UNFPA. Then came the 1994 election, ushering in a conservative majority in Congress. Three months ago, the House voted to turn back the calender and halt funding to UNFPA. Before its summer recess, the Senate Foreign Relations Committee ignored the House and authorized $35 million for the fund - no mean feat considering that chairman Jesse Helms (R) of North Carolina is an avowed foe of United Nations and population spending. On Sept. 12, the Foreign Operations Subcommittee of the Senate Committee on Appropriations will be the next stop for the foreign assistance bill and the fate of US funds for UNFPA. Two days later, the full committee is scheduled to mark up the bill. It is hoped that when the smoke has cleared, Appropriations will have followed the lead of the Foreign Relations Committee by recommending that funding for UNFPA be maintained, with the segregated-account safeguards the authorizing committee approved. With a strong clamoring in both major political parties to reduce spending, cuts in foreign assistance are a virtual certainty. But they should be fair. The Senate would be wise to distribute development assistance next year at the same percentage of total overseas spending that these programs currently hold. Finally, it will be important for population- stabilization advocates on the Appropriations Committee to exert leadership - especially on the Republican side of the aisle. It may not be enough for such longtime supporters as chairman Mark Hatfield (R) of Oregon and committee members Arlen Specter (R) of Pennsylvania and James Jeffords (R) of Vermont to stand up and be counted. They must stand up and be heard.