A vigorous international campaign is mounting against plans by Mexico and Japan’s Mitsubishi Corp. to build the world’s largest salt plant on a Pacific lagoon that is a major breeding ground for gray whales. The proposed saltworks on the San Ignacio Lagoon in Baja California would pump 6,600 gallons of water out of the lagoon each second and flood more than 116 square miles to make evaporation pools. The Mexican government rejected plans for the project in 1995, but project backers are reworking their proposal and are expected to formally resubmit it this fall. On Monday, a letter from 33 renowned scientists denouncing the project appeared in a two-page ad in a Mexico City newspaper, and yesterday a similar full-page ad appeared in the New York Times.