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Mr. Wang, the food exporter, is a card-carrying member of Ms. Tsai’s party but is actively canvassing for Mr. Ma. He worries that a Tsai win might cause Chinese buyers to halt the milkfish purchases that last year amounted to $4.5 million - about $45,000 for each of the 100 Xuejia families selling to the mainland.

“The Chinese would be most disheartened if the election outcome showed that all their goodwill gestures were ignored by Taiwanese,” he said.

In April, China’s state-run Shanghai Fisheries General Corp. placed a trial order with Xuejia fish growers for 1,800 tons of milkfish, a local staple that tastes a bit like trout and until recently was unknown on the mainland.

To sweeten the deal, the company provided the 100 families with loans to buy baby fish from other producers and equipment to engage in large-scale fish farming.

Fish farmer Hsieh Chin-San, another Ma supporter, said the 25 percent premium that mainland buyers were paying for milkfish is crucial to his well-being.

“It assured a profit during harvesting season no matter how far any oversupply causes prices to fall,” he said.

Even so, Xuejia milkfish farmers said voting for Mr. Ma would not soften their opposition to a Chinese takeover of the island.

“Brothers can have different views, and the Chinese still cannot force unification on us,” said Lin Li-Chu, a stocky fisherwoman of 50 who is hooked on TV political talk shows and considers herself a strong Taiwanese patriot.

The tariff reductions have come on top of China’s continued major purchases of electronic parts, such as cellphone chips and television panels, which make up the bulk of Taiwanese exports to China.

For Chinese President Hu Jintao, who is set to leave office later this year, a defeat for Mr. Ma would be a big blow to his legacy and embolden harder-line politicians and military leaders, for whom the Taiwan issue is crucial.

So worried is Beijing that overt favoritism for Mr. Ma might backfire that it has tried to keep Chinese media from traveling to Taiwan and prevent scholars both from going to the island and talking to reporters.

“The boss says we’re not giving interviews before the election finishes and the results come out. It is too sensitive for us to talk right now,” said Peng Wenxue in the research office of the Institute of Taiwan Studies at the Chinese Academy of Social Sciences in Beijing.

On the broader unification issue, Beijing seems far from winning minds. According to the last government survey on national identity, taken in 2009, 65 percent of island residents regard themselves as Taiwanese, against 11 percent as Chinese. The rest had no opinion or saw themselves as both.

Oyster farmer Wang Chang-hao from Tainan said he wants commercial cooperation with the mainland and little beyond it.

“We Taiwanese shouldn’t hate the Chinese,” he said, “but rather work together in the global village.”