(FORTUNE Magazine) – Despite Reagan Administration pleas to give its ''constructive engagement'' policy a chance, Congress moved closer to imposing tough sanctions against South Africa's white minority government. The House voted 295 to 127 to halt new U.S. bank loans to the South African government, bar new investment by U.S. companies, stop the sale of computer technology, and cut off the importation of Krugerrands. Similar measures were approved by a surprising 16- to-1 vote in the Senate Foreign Relations Committee but face harder going with the full Senate. Congress's hard line may have been one reason the Administration recalled the U.S. ambassador to South Africa in mid-June to protest military incursions into neighboring countries. Both the South African government and U.S. corporations have opposed sanctions, partly on the ground that they will have little real effect on apartheid. But proponents of sanctions say that efforts by U.S. companies to improve the economic conditions of South African blacks are not the issue. ''Black South Africans are struggling to achieve political enfranchisement,'' says Randall Robinson, executive director of TransAfrica, a lobbying group that organized demonstrations at the South African embassy in Washington last fall and brought the apartheid issue into the national spotlight. The talk of sanctions is already affecting the gold coin market. Krugerrands, which normally carry a 3.5% premium over the price of gold, lost two-thirds of that premium after the House bill passed, apparently because of fears they would become harder to sell. An import ban could help Mexico and Canada, which have been trailing South Africa in efforts to sell gold coins in the U.S. James Corkery, Canada's Master of the Mint, says a ban would help increase his country's estimated 40% to 45% share of the U.S. gold coin market.