By Duncan Kennedy
BBC News, US-Mexico border
The border fence stretches for hundreds of kilometres
Of all the issues in this year's US presidential election, immigration is the one that touches the rawest of Democratic and Republican nerves.
After last year's failure by President George W Bush to get his comprehensive immigration plan past Congress, it has become fertile and divisive ground for candidates in the race.
But there is one area of immigration policy that is proceeding, despite the political stalemate: the building of the border fence between the US and Mexico.
Hundreds of kilometres are under construction along the US's southern frontier.
Estimates for the cost of the project have ranged from $2bn to $10bn (£1bn to £5bn).
Flying by helicopter some 100m (330ft) above the fence, it can sometimes be hard to see.
In the section along Arizona's border with Mexico I went to examine, the barrier appears like a thin black line snaking along the desert floor below.
Once you fix your eyes on the line, it becomes clear this is one area where building is racing ahead.
Roughly a mile of fencing is being erected every month here.
Areas of once-untouched desert are now disappearing under a lengthening slice of man-made fencing.
After setting down, we were able to get up close to the men and machines making the new fence.
Coast to coast, from the Pacific to the Gulf of Mexico, some 300 miles (500km) of barrier are completed, with another 700 miles (1100km) set to go up by the end of this year.
The fence itself is an impressive, sun-blocking, engineering feat.
Agent Jose Gonzalez of the Arizona Border Patrol tells me each 4-metre-high (13ft) panel can withstand a car impact at 45mph (70 km/h).
"It's been tested using the military's armoured vehicles," he says. "We think it will withstand pretty much any migrant car or truck."
In other places, where the terrain is more suited, electronic sensors, not walls, are being installed.
Illegal crossings are down 60% at some areas of the border
But whatever "asset" is being constructed, to use Agent Gonzalez's term, it all raises the same question of whether it will work.
"It won't stop everyone," is his honest answer.
"But we believe most migrants will be deterred".
Agent Gonzalez later drives us parallel to this gigantic metallic barrier. You can see through its grey bars. Just across the ravine inside Mexico we spot a man.
When he sees the fence and us, he changes his mind about crossing and runs off.
He is not the only one deciding he needs a plan B. In some places where the wall has been completed, and where extra border patrols are in place, illegal crossings are down by as much as 60% compared with a year ago.
The days when Mexicans, Nicaraguans, Salvadorians and others could step over flimsy strips of barbed wire to begin a new life in the US are now numbered.
'Just for the US'
The fence is part of President Bush's attempts to convince Congress he is tough on immigration.
Congress gave him the go ahead for his fence but not his policy on dealing with the estimated 11 million undocumented migrants already in the US.
That unfinished business has turned into a key issue in this year's presidential race.
In some states, it is the social topic of the 2008 campaign.
We later cross into Mexico to find the fence is just as controversial, but for very different reasons.
Here, it has been likened to the Berlin wall.
"What do you think of it?" I ask Marco, a Mexican, deported from America and now trying to get back into a country where hourly wages are up to 10 times those in Mexico.
"It's unfair" is his simple reply.
Marco stands dwarfed by the new border fence in front of him, but not, it seems, by the task ahead.
"Some will slip through," he says, "and I hope to be one of them."
We then climb inside one of the orange pick-up trucks used by a migrant help group, Grupo Beta.
The group, set up by the Mexican government, takes us along the fence on the Mexican side. We go past migrants straining to look up at the immense structure before them.
Enrique Enriquez from Grupo Beta tries to be diplomatic when I ask him what he thinks of the wall.
"It's fair for them, it's fair for the United States," he says.
"But maybe it's unjust this side. It's for the protection of the United States. It's just for them."
Opinion polls in the US suggest this belated attempt to physically halt unchecked immigration is popular.
Many millions of figurative horses may have bolted, but this barn door is now being very firmly shut.
Less clear is what to do with those migrants who have already made it through illegally. The outcome of this year's election may help decide that.
For now, the US is a country that is putting in place a border, but not yet a policy.