The North American free trade agreement cleared its last legislative hurdle when the Mexican Senate overwhelmingly voted its approval. The trade pact, which will link the United States, Canada and Mexico in a $6.3 trillion free trade zone scheduled to go into effect on Jan. 1, 1994, won late Monday by a margin of 56-2 after nearly 12 hours of speeches. The big victory was not unexpected since the governing Institutional Revolutionary party of President Carlos Salinas de Gortari holds 61 of 64 seats in the Senate. The Canadian Parliament approved the pact in May, but Prime Minister Jean Chretien has said he wants some parts reworked before he will proclaim it. Chretien wants better rules on what constitutes a subsidy and what sort of sanctions should be imposed for trade violations. He also wants the same protection for energy resources that Mexico won for its oil industry.