GUADALUPE VALLEY, Mexico — The doors were locked. The lights were out. When Hugo D’Acosta and 60 of his neighbors reached Ensenada City Hall after being tipped off to a nighttime vote that would open their beloved wine region to Florida-style housing and golf courses, they had to shout just to get in.

But it was no use: In a fourth-floor meeting room, lawmakers quickly voted to permit urban and suburban development in the agricultural heart of northwestern Mexico, the Guadalupe Valley, despite angry opposition from those who have spent decades making it an international destination for wine, food and quiet.

“It will destroy everything,” said Mr. D’Acosta, 55, one of the valley’s premier winemakers. “We can put up plastic grapes to make it look pretty, but that’s it.”

Municipal council members argue that the new zoning regulations will preserve the valley and increase property values, spreading out the benefits of a boom. But the new rules subvert the state-approved regional plan they were supposed to clarify by allowing up to 10 times as much housing density while significantly weakening public oversight. Independent scientists say the arid valley simply cannot sustain the intensified development, creating what many here see as a threat to a national treasure and a vital test of Mexico’s young democracy.