NOGALES – Alejandro Vega hiked five days through the Arizona desert and then toiled 10 years busing restaurant tables, building roads and cleaning manure out of horse corrals in the United States before his deportation in 2009.

Now, facing the southern side of a 20-foot-tall copper-hued fence in the border city of Nogales, Mexico, he says he’s ready to risk prison or death to get back in.

“I don’t care how many times I need to try,” said Vega, 38, who in March scaled the barrier’s iron slats and sprinted to a Walmart parking lot only to be caught and expelled again.

“My life is there — there’s nothing for me in Mexico,” he said. “Everything has its risk, but if you never risk, you never gain.”

The daily struggle along the rugged Nogales frontier, which the U.S. government ranks as the highest-risk sector of its border with Mexico — a region where 120,000 people were caught crossing last year — points to a security challenge central to enactment of any new immigration law. Senators are advancing a bill requiring that the Border Patrol show “90 percent effectiveness” in securing this and other high-risk border sectors — areas where more than 30,000 people a year are caught crossing — before legal rights are conferred on the estimated 11 million undocumented immigrants in the U.S.

The concern about border security, which Republican leaders call essential to a broader agreement on a path to citizenship for the undocumented, visas for guest-workers and farmworkers.

In the House, where the immigration bill faces long odds, Judiciary Chairman Bob Goodlatte, R-Va., calls border security “very crucial” to any plan. “Exactly how it works in conjunction with the rest of immigration reform, it has yet to be decided,” he said.

Legislation filed by a bipartisan group of eight senators demands a border-control plan with fencing and surveillance assuring that 90 percent of those who attempt to cross are caught or turned back to Mexico in these high-risk sectors before other steps are taken on immigration.

There are three such sectors: The area south of Tucson, Ariz., that includes Nogales; the border near Laredo, Texas; and the Rio Grande River valley near Brownsville. The effectiveness of security last year, according to a Government Accountability Office report based on Border Patrol data, has ranged from 87 percent in the Tucson sector to 71 percent along the Rio Grande.

Senators say this makes the border-security in their plan obtainable, enabling the government then to move forward with citizenship for the undocumented and other measures.

“The border-security triggers are strong, but achievable,” Sen. Charles Schumer, D-N.Y., who has visited the Arizona border in negotiations over the bill, said at an April 18 Washington news conference announcing it.

THE BUSIEST SECTION

In the desert region south of Tucson that alternates between rocky gulches and 7,000-foot peaks, part of a 262-mile stretch of an almost 2,000-mile-long border, the challenge is spelled out in numbers: In this sector alone, 124,363 people were caught in 2011, the GAO reports. That’s close to one-third of the 328,000 apprehensions along the entire Southwest border. Another 43,539 were turned back; an estimated 25,376 got away.

Manuel Padilla, chief patrol agent of the Tucson sector, said calculating the effectiveness rate, which only applies in the border areas between ports of entry, is “not an exact science.”

“In the urban areas, we have a very high effectiveness rate,” he said. “Once you start getting into the rural environment, that’s where it gets more difficult.”

On Interstate 19, at the “19 Charlie” checkpoint between Tucson and Nogales, sits a white, warehouse-sized, flood-lit canopy crossing three lanes about 20 miles north of the border. Agents with K-9 dogs scan a line of cars for suspicious behavior. They target shuttle vans, pulling over many.

“Every day, we have a seizure of some kind at this checkpoint,” said Leslie Lawson, patrol agent in charge of the Nogales station.

In the desert surrounding the checkpoint, cameras and infrared scopes detect illicit movement. In the days after footprints and other evidence of illegal crossing are discovered, agents work to match up the information with the immigrants they apprehend to determine their effectiveness rate.

Officials in Texas’s Rio Grande valley haven’t had as much success in stemming illegal entries. While they’ve raised the sector’s effectiveness rate from 55 percent in 2006, it remains the major area where migrants are most likely to successfully enter the U.S., the GAO reported.

CLOSE QUARTERS

It isn’t only desert that toughens the task.

The Tucson-sector border slices through urbanized Nogales, where homes in Mexico and the U.S. stand a few dozen feet apart. In Nogales, 2.5 miles of rust-colored bollard fencing with iron posts sunk several feet deep divide a community in two. Even the sewage pipes must be patrolled, and tunnels filled.

Lawson, spotting a lookout on a Mexican hilltop, predicts a crossing soon. Within the hour, her truck radio crackles with an apprehension.

“It is a long, slow process, and it is not going to happen overnight,” Lawson said of the battle against illegal immigration. “As we’re gaining control in the urban areas, they move to the flanks.”

Outside Nogales, the border dips and rises over the rolling landscape invisible to strategically placed cameras. To the west, in the Tumacacori Highlands, mountain peaks block vehicular access, limiting access even by all-terrain vehicles or horses and forcing agents to hike in on foot. Padilla said it will require an infusion of technology such as sensors and cameras to enhance enforcement in these outposts.

There’s a practical limit to barriers that can be built. It costs $6 million a mile to fence flat land, Lawson said, and more on rougher terrain. “Is a 15-foot fence on top of a 5,000- foot peak going to make a difference?” she asked.

Overall, attempted border crossings are down since 2000, when 1.68 million people were apprehended on the Southwest border, according to the U.S Customs and Border Protection agency. Last year, the number was about 357,000.

ECONOMIC INCENTIVES

For all the security, the economy has played a role in the immigrant flow. The U.S. recession and Mexican growth have lessened the lure of manual labor north of the border, according to a Pew Research Center study released a year ago.

Net Mexican migration dropped to zero from 2005 to 2010, it found, with the 1.4 million people immigrating to the U.S. equaling the number moving from the U.S. to Mexico. That trend could reverse should the Mexican economy slow or U.S. activity pick up.

“This is the most opportune moment for migration pressures,” said Doris Meissner, commissioner for the U.S. Immigration and Naturalization Service under President Bill Clinton. The agency was responsible for border patrol until the Sept. 11, 2001, terrorist attacks, later divided into three new entities under the Department of Homeland Security.

The Texas border has faced more pressure because it’s the preferred route for immigrants from Central America, Meissner, a senior fellow at the Washington-based Migration Policy Institute, said in a telephone interview.

While the Mexican economy has grown at about double the pace of the U.S. economy since the end of 2009 amid an explosion of manufacturing, nations like El Salvador and Honduras lacking that dynamic have been plagued by drug wars and gang violence.

“The bigger back story is the things in Mexico that have changed this equation,” Meissner said. “The difficulty is that they apply to Mexico and not Central America.”

Nogales Mayor Arturo Garino questions if anything will ever completely secure the border – and if it’s worth it.

“I don’t care what kind of a fence you put – if somebody needs to feed their family, and they think the opportunity is coming to the United States, and they still know that they will get a job, that nobody’s going to be asking questions about the job, they will live in the shadows,” he said. “I’m sure you would do it, and I would do it too, if we needed to survive.”