WASHINGTON — The U.S. fence along the Mexican border is less a wall than a stuttering set of blockades: half barrier, half gaps.

Americans are split pretty much the same way: half in favor, half against, passionate on both sides when it comes to the idea of erecting a wall to keep people from entering the country illegally.

It can seem a shaky foundation as the United States rushes to complete the fencing on nearly 700 miles of the border by the end of the year. That’s when a new administration arrives in the White House with its own ideas about security, freedom, the 11 million illegal immigrants already here and the prospect of many more on the way.

Nearly half complete, the multibillion-dollar fence project stretches from the Pacific surf at Tijuana to the Gulf of Mexico near Brownsville, Texas. The messages it sends are decidedly mixed.

For Rep. Peter King, the New York Republican who wrote the legislation to build the fence, the message is simple: Don’t sneak into America; we are taking control of our borders.

For others, the fence is inconsistent with a country founded by immigrants and priding itself on opportunity.

Homeland Security Secretary Michael Chertoff says it’s simply a new law enforcement device, part of a multipronged crackdown on the flow of illegal immigrants. The government also has hired more border agents, stepped up enforcement nationwide and increased penalties for those who don’t follow the law.

“I don’t invest the fence with the iconic significance that some people place on it,” Chertoff says. “To some people, it is a be-all and end-all of controlling the border. To some people, it is a symbol of ... the Berlin Wall. I think it’s a tool.”

The concept of a border fence took on new life after the Sept. 11, 2001, terrorist attacks, which revived the heated immigration debate. Intelligence officials have said the holes along the southwest border could provide places for terrorists to enter the country.

About 317 miles of the southwest border fence have been built, with plans for another 353 miles by the end of the year. Longer term, there are plans for physical fencing or surveillance and detection technology along the entire 2,000-mile border by 2010.

An Associated Press-Ipsos poll last month found Americans just about as split as they could be: 49 percent in favor of the fence, 48 percent opposed. Tellingly, a majority of 55 percent think it won’t fix the problem.

Congress has allocated $2.7 billion for fence construction, and there’s no estimate how much the entire system — the physical fence and technology — will cost to build, let alone maintain.

The new construction includes completion of a nearly solid stretch from San Diego to Yuma; a new section extending several miles in each direction from Lukeville; additional lengths flanking Nogales, Ariz., and Columbus, N.M.; extension of the current barrier at El Paso; new sections near the Texas border towns of Esperanza, Presidio, Del Rio and Eagle Pass, and a dotted line of fence stretching from Roma to past Brownsville.

Border fences have been sprouting across California, Arizona, New Mexico and Texas for decades — dating to the 1940s, when the International Boundary and Water Commission built 234 miles of fence to keep out foot-and-mouth disease.

As a result, the U.S. fence is a patchwork of old and new construction and in varying states of repair; the only consistency is a uniform ugliness.

In San Diego, rusted, corrugated metal wades ashore from the Pacific onto a beach and becomes a 9-mile wall that dips into canyons, runs along hillsides and beside a highway. In Arizona, short vertical posts, some connected by horizontal rails, mesh fencing and World War II surplus corrugated steel sheets are scattered along the border from Yuma to Douglas. In New Mexico, 15-foot poles poke up from the desert floor on either side of the Columbus port of entry, rust-colored pipes just inches from each other, allowing enough space to wriggle a hand between. And in Texas, dull gray panels of thick steel fencing curve along the Rio Grande through downtown El Paso, patched here and there with mismatched pieces of metal.

Over the years surveillance cameras, ground sensors and unmanned aerial drones have been used in spots along the border. But the current building spree is the first comprehensive federal push to seal the entire stretch with either physical fencing or detection and surveillance technology.

The path hasn’t been smooth. The government’s first test of a 28-mile “virtual” fence — a $20 million combination of cameras, satellite images and relay towers — was a disappointment. The technology was designed to distinguish people from cattle 10 miles away and improve border monitoring. But Border Patrol agents weren’t in on the planning of the system, and in practice the components didn’t work well together, sending contractor Boeing back to the drawing board.

The project has had other hiccups.

The fencing plan affects about 480 landowners. Some citizens are faced with moving out of their homes and selling their property to the government because the placement of the fence would significantly affect the value of their properties. Others could accept a government payment as compensation for reduced value.

Environmentalists have said the fence puts already endangered species such as two types of wild cats — the ocelot and the jaguarundi — in even more danger. The fence would prevent them from swimming across the Rio Grande to mate.

Anticipating these challenges, Congress gave the department the authority to bypass certain environmental laws and condemn private property. And Chertoff has taken full advantage of that power, waiving 40 laws that had delayed 528 miles of fencing. Some Democrats are challenging Chertoff’s use of these waivers, saying the blanket actions are unconstitutional.

In an effort to smooth the way, federal officials have held more than 100 meetings with lawmakers, environmental groups and residents. But Chertoff, a former prosecutor and judge, says, “We listen, but we don’t view it as an opportunity to endlessly kick the can down the road.”

All three major presidential contenders supported legislation that called for building the fence, but Sens. Hillary Clinton, D-N.Y., and Barak Obama, D-Ill. have softened their positions when the topic has come up during the campaign. Both now say they will listen more to landowners who object to the fence.

Critics say the barriers are penetrable and the surveillance technology of the high-tech virtual fence is easy to evade. Arizona Gov. Janet Napolitano once observed, “You build a 50-foot wall, somebody will find a 51-foot ladder.”

Building fences is not a new idea, and people who desperately want to get into the United States will always find ways around the barrier, said Adrian Lewis, chairman of the history department at the University of North Texas. He cites the heavily armed demilitarized zone between North and South Korea, where he says people continue to get through by tunneling underground. The Maginot line, built by the French after World War I to stop German tanks, merely forced them to take another route.

Fences, Lewis says, are placebos. “It makes people feel good. It doesn’t really do anything.”

The Homeland Security Department counters that its plan is already working. And the number of people they’ve caught trying to cross illegally has gone down, meaning fewer people are taking their chances now that the barriers are in place.

King, the New York lawmaker who supports the fence, said there’s no way to ensure that the next administration will support it. He said it continues to be imperiled by the feeling among some that the fence sends the wrong message to the world. “It’s going to be an ongoing struggle,” he said.

Chertoff agrees.

“All I can do is carry the ball as far down the field as I can while I’m in the game,” he said. “I’ve got to hand it off to somebody else.”

ASSOCIATED PRESS writers Traci Carl and Elliot Spagat in California, Art Rotstein in Arizona, Alicia Caldwell and Christopher Sherman in Texas contributed to this report.