Mitt Romney and Newt Gingrich have promised to complete a nearly 1,950-mile fence. Michele Bachmann wants a double fence. Ron Paul pledges to secure the nation's southern border by any means necessary, and Rick Perry says he can secure it without a fence - and do so within a year of taking office as president.
But a border that is sealed off to all illegal immigrants and drugs flowing north is a promise none of them could keep.
'Securing the border is a wonderful slogan, but that's pretty much all it is,' said Ted Galen Carpenter, a senior fellow at the libertarian Cato Institute. 'Even to come close would require measures that would make legal commerce with Mexico impossible. That's an enormous price for what would still be a very leaky system.'
Perry, the longest-serving governor of a state that makes up roughly 65 percent of America's border with Mexico, already knows that. What he's actually pledging, clarifies spokeswoman Catherine Frazier, is achieving 'operational control' of the border - defined by the U.S. Border Patrol as areas where it can detect, respond to and interdict illegal activity either at the border or after entry into the U.S.
The U.S. Border Patrol says 873 miles of the border, about 44 percent, have been brought under operational control. Homeland Security Secretary Janet Napolitano has said that 'the border is better now than it ever has been.'
Still, that means full control isn't even half met. And even getting this far required bolstering the ranks of the Border Patrol to the highest levels ever, from about 9,500 along the border in 2004 to 18,152 today. Immigration and Customs Enforcement also has a record number of agents on the border, and five Predator drones now patrol strategic parts of it, with a sixth coming by the end of the year. About 650 miles of fencing has been constructed, and 1,200 National Guard soldiers dispatched last year to Texas, California, Arizona and New Mexico have had their deployment extended through the end of the year.
Campaigning in Iowa last week, Gingrich signed a pledge to build a fence stretching the length of the border by the end of 2013. That may help him recover from a recent statement that illegal immigrants who have been established in the U.S. for many years should be allowed to remain in the country - a position his opponents have likened to amnesty.
Perry has steadfastly opposed the fence, saying it would take 10 to 15 years to build, cost $30 billion and wouldn't work anyway. Instead, he wants to flood the border with more National Guard troops until the number of Border Patrol agents necessary to really secure the area are trained and deployed. He also wants to build strategic fencing in high-traffic areas and make better use of airborne surveillance. Perry claims that would mean full operational control by January 2014.
Romney, meanwhile, has publicly agreed with Perry that tackling larger immigration policy reform is impossible without first securing the border.
By some measures, U.S. authorities already have made strides toward that goal. The Pew Hispanic Center says the number of illegal immigrants in the United States peaked at 12 million in 2007, but then dropped by almost 1 million through 2009, and has largely held steady since then at about 11.1 million.
Border Patrol apprehensions of illegal immigrants have also fallen sharply. In fiscal year 2011, which ended Sept. 30, the Border Patrol captured 327,577 illegal immigrants on the southwestern border - the lowest total in four decades.
The poor U.S. economy makes would-be illegal immigrants less likely to come, andthose who do must contend with Mexico's drug war, which has seen cartel gunmen slaughter people heading north and dump their bodies in mass graves. Jeff Passel, the Pew Center's senior demographer, said the trip is now so risky that the number of illegal immigrants using pricey people smugglers has spiked.
'It's hard to separate the effect of the economy and increased enforcement,' Passel said. 'It's a lot harder physically to get across the border, but it's also more expensive and more dangerous, and you're faced with the prospect of having no job when you get here.'
Spillover into the U.S. of Mexican drug violence is also difficult to measure. In terms of violent crime, El Paso, Texas, ranks among the safest cities in the U.S. - even though it's across from violence-torn Ciudad Juarez. Drug crime aside, Rep. Michael McCaul, a Texas Republican who heads a Homeland Security subcommittee, said he's worried about cartels teaming with international terrorists.
'It's not secure,' McCaul said of the border, 'and anybody that lives down there, I think, will tell you that.' U.S. intelligence officials counter that they know of no case in which a terrorist has sneaked across the border to plot actively against the U.S.
Carpenter, who has written extensively on the increasing brutality of Mexican drug cartels, called the presidential candidates' pledges to secure the border 'mainly defensive.'
'If you don't take a strong position on border security, you leave yourself open to allegations that you're soft on immigration or drugs,' he said.
Michael Lytle, a former consultant on border security and counterterrorism, said it's hard to even conceptualize a fully secure border since the Arizona desert presents different challenges than the millions of commercial trucks rumbling north into Laredo, Texas, or than pedestrians streaming from Tijuana to San Diego. Tracking would-be terrorists also has little to do with stopping migrant workers sneaking into the U.S., or coping with well-armed drug smugglers.
'You can't look at it as 'the whole border,'' he said.
Lytle, an associate professor at the University of Texas at Brownsville, said a deployment of 15,000 National Guard troops could make an impact - but it would be a hard sell for a Defense Department facing budget cuts.
'A troop surge there, would that seal the border? Probably not,'; Lytle said. 'And even if it did, how long could you sustain that?'