NOGALES, Ariz. (AP) — The fence rises from the rock and hardscrabble of the desert floor, a formidable 15-foot-high curtain of corrugated metal that stretches into the mirage of heat and distance. Newer sections feature 20-foot high steel columns, deeply planted, narrowly spaced, so no human slips between.

The start-and-stop span — 646 miles long — has become a fierce polemic, a bumper sticker, a popular backdrop for campaign commercials during an election year with another sulfurous immigration debate.

The best known TV spot features Republican Arizona Sen. John McCain kicking along a dusty road in this hilly border city, fuming to his companion, the Pinal County sheriff, about drugs and immigrant smugglers and kidnappings. Wearing his Navy baseball cap and squinting into the sun, McCain could be rounding the corner to the gunfight at the OK Corral.

"Complete the danged fence," he spits, his jaw drawing into a knot.

The government has spent $2.4 billion since 2005 to build the fence as it presently stands. And the prevailing political sentiment would appear to be, build it faster and higher.

But what McCain and other politicians often fail to point out is there's no shortage of ways to get past the fence. Immigrants scale it with ladders. Smugglers use blowtorches and hacksaws to penetrate it. They use trucks with retractable vehicle ramps to roll pickups full of marijuana over the fence. They knock down vehicle barriers and erect lookalikes that are made out of cardboard and easy to move.

When backed up by border agents and surveillance technology, the fence can help reduce immigrant traffic or redirect it to other locales. But even some advocates for tougher enforcement say it's unclear whether the fence cuts the overall number of illegal crossings.

"The whole point of the fence is to work in concert with other things, but, by itself, you can't expect it to be the end-all and be-all," said Steven Camarota, research director for the Center for Immigration Studies, which advocates for strict immigration laws.

An estimated 45 percent of America's 12 million illegal immigrants came here legally on various visas or border crossing cards and remained after their legal stays expired. The fence couldn't have stopped that. And the fence doesn't directly confront employers who fuel illicit crossings by hiring illegal immigrants.

Even so, at least one candidate in nearly all of Arizona's top political races, including McCain's, touts the fence as essential, or uses images of the barrier in campaign materials.

The fence covers about 30 percent of the 2,000-mile U.S.-Mexico border, about one-third the length of the Great Wall of China. Nearly half of the fence sits in Arizona, the busiest gateway for both illegal immigrants and marijuana, with the rest equally divided in California, New Mexico and Texas. The government plans to build 6 more miles of fence by year's end.

The Border Patrol declined to say how many times it has recorded fence breaches, but a government audit released last year reported thousands. Each breach costs an average of $1,300 to repair. On top of the price tag for building the fence, it will cost another $6.5 billion over the next 20 years to maintain it and related equipment.

Politicians focus on the fence, advocates from both sides of the debate say, because of its symbolic value. It allows voters to envision the government's security efforts, and it's a shield against the violence on the Mexican side.

The focus on the fence by political campaigns appears to be isolated mostly to Arizona, where a law set to take effect July 29 will order police to question anyone they suspect of being in the country illegally.

A gubernatorial candidate in New Mexico also used the fence as a backdrop in a campaign video. And just a day after entering Washington state's Senate race against powerful Democrat Patty Murray, Republican Dino Rossi said Thursday he wants to see a "tall fence with a high gate" for those who look to enter the U.S. illegally.

In Arizona, support for fence-building or images of the barrier can be found on the websites and campaign literature of at least one candidate in six of the state's eight congressional races, the U.S. Senate contest and races for governor and attorney general.

Ben Quayle, a Republican candidate for an open congressional seat and son of former Vice President Dan Quayle, calls for building a continuous border fence as part of a broader strategy. One of Quayle's competitors in the Republican primary, Ed Winkler, wrote an essay about his recent trip to the border and posted four photos of himself standing near the fence.

Democratic U.S. Rep. Raul Grijalva said on his campaign website that the country's immigration woes won't be solved by building the fence and sending troops, but rather by a comprehensive immigration overhaul.

Border Patrol spokesman Lloyd Easterling said the fence represent one of several tools used in reducing the number of immigrant arrests by more than 50 percent over the last five years. Other factors include 8,000 new Border Patrol agents, more technology and America's economic downturn.

President Barack Obama on Tuesday promised to send 1,200 Guard troops to the border to support efforts to block drug trafficking and temporarily supplement Border Patrol agents until more agents can be trained. McCain says more are needed — he tried unsuccessfully Thursday to get 6,000 more troops — arguing that the security situation along the border has deteriorated so badly that 3,000 guard troops are needed just to help protect his state.

In Nogales, the shoe stores, duty-free shops and other downtown businesses are just yards from the corrugated barricade, which has welding scars where breaches have been patched.

Forty border tunnels have been discovered in Nogales since the mid-1990s.

Surveillance cameras, sensors and trailers carrying stadium lights help Border Patrol agents who monitor the fence. Deeper into Nogales, layers of agents are placed to try to stop immigrants who slip past the border.

Mayor Octavio Garcia-von Borstel said the fence is essential to maintaining the quality of life in his city of 24,000. "With no fence, they would be free (to run through)," he said.

More than 100 miles away in the southeastern Arizona city of Douglas, Mayor Michael Gomez said the fence "is not going to work. They jump right over it."

But he believes there aren't enough Border Patrol agents there to back up the fence. At night, ultralight planes used by drug smugglers buzz right over.

Immigrant rights advocates say the fence prompts migrants to cross in remote areas where they face dangerous, often deadly obstacles on their way to a better life.

"It's a very big problem," said Walter Ewing, a senior researcher for the pro-immigrant Immigration Policy Center. "It won't be fixed by fences. It centers around economics."


Associated Press Writer Rachel La Corte contributed to this story from Seattle.