But residents on both sides of the fence say the barrier already has jarred their communities. It is difficult to find anyone who does not have a relative or close friend across the line. Many make regular walks across the border to visit or attend parties. Some Mexican residents receive mail at the tiny post office in Jacumba because their town has no post office. Beer sales at Jacumba's two stores are brisk, in large part because alcohol sales are barred in Jacume, part of a state-sponsored communal farm. And it is a common practice for Jacume residents to keep a car on each side of the border, which is blocked to most traffic, and tote across everything from donated bicycles to bags of cement.
In addition to Lopez, other Jacume residents with green cards have relocated to the U.S. side so their children can attend local schools. At least one has even joined the local Kiwanis, said Felix Bachmeier, owner of the Jacumba Hot Springs, a spa that is Jacumba's main tourist draw. Bachmeier employs three Jacume men with work permits who make the crossing daily.
"Most Jacumba people know the Jacume people," Bachmeier said. "They are here to support the local stores. A lot of work is done by them."
Mario Ramirez, 36, a handyman at the spa, grumbled that a sealed border would force him to quit his job or break the law. The laborious drive through Tecate is out of the question, he said.
Ramirez, whose family owns a small restaurant in Jacume, said border closure also could shut out the American customers who make up much of its business.
"It's all finished. My business is finished. My work is finished," Ramirez said. "What else can we do in Jacume? There is nothing."
The hubbub has shaken the towns from a sleepy obscurity. Although Hollywood movers once flocked to Jacumba for its hot springs, the town has been a forgotten country crossroad since Interstate 8 passed two miles to the north more than 20 years ago. Jacume--a sunbaked grid of family farms, with an industrial henhouse and a miniature rodeo--sits seven miles from the nearest highway, midway between Tecate and Mexicali.
Experts said the volatile immigration issue is sure to draw increased attention to towns that similarly have grown up as cross-border sisters. The 2,000-mile border with Mexico is dotted with rural twin communities bound closely by blood ties and business, particularly in Texas and New Mexico, said Larry Herzog, a professor at San Diego State University who has written a book on border life.
"We have to start thinking about all the twin settlements along the border . . . as part of not two individual places but single functional regions that are joined economically and joined functionally," Herzog said.
Another border scholar said Jacumba and Jacume are feeling the effects of a historical cycle of shifting border policies spurred by economic crises, political winds and U.S.-Mexico relations.
"These big macro events impact on these little communities and change people's worlds," said Anibal Yanez, who teaches geography at Cal State San Marcos.
The change already felt in Jacumba and Jacume will only become more acute. With a planned boost in the number of local agents, the patrol chief said he can enforce the ban on even casual crossings by Jacume residents once the fence is complete by the end of the year.
"Over the years, when they cross the lines they're in violation of customs laws. When we get our fence project done, we'll be better able to address these people," Dierkop said. "They know well it's against the law."
Until then, locals will debate the new fence's likely effects even as they continue to walk past the old dilapidated one.
Raul Gallego Jr. and his wife, Monica, who live on the U.S. side, were in Jacume one day last week to deliver tomatillo plants and burn trash on land owned by Monica's family. The husband downplayed the fence's importance, predicting that people who wanted to cross would find a way over or around it.
But, as the couple bumped along a rutted Jacume road in a aging Chevy pickup, Monica said the steel barrier was breeding bad vibes. "It's like they're separating Jacumba and Jacume. Before we were always free to go back and forth. But now it's like," she paused, "a prison."
As the couple drove on, reminiscing about the Jacume farm country of their childhoods, signs of the town's recent troubles were close at hand.
Raul Gallego Jr. turned down a tiny side road, startling a group of more than a dozen men jammed into the back of a stalled pickup with California license plates. The men were not from Jacume, he said, and probably planned to sneak across the border once it got dark.
A cheer erupted as the truck sputtered to life and headed deeper into the countryside, kicking up a cloud of dust.
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A fence to deter smuggling of illegal immigrants is being built between the east San Diego County town of Jacumba and its twin on the Mexican side, Jacume.