Arrests of people trying to sneak into the US from Mexico have plunged to the lowest level in four decades, the latest sign that illegal immigration is on the retreat even as legislatures, Congress and presidential candidates hotly debate the issue.

Behind the historic drop is a steep decline in the birthrate in Mexico and greater opportunities there relative to the weak US economy. Stepped-up US patrols along the border make it both riskier and more expensive for Mexicans to attempt to enter the country.

Government crackdowns on US employers who hire illegal workers also have discouraged immigrants. The Supreme Court agreed Monday to decide whether an Arizona statute targeting illegal immigrants interferes with US law.

The decline in Mexican immigrants is being felt as far away as farms in Washington and Michigan, which weathered labor shortages during the recent apple harvest.

The US arrested 340,252 migrants along the Mexico-US border in the fiscal year that ended Sept. 30 -- down 24 percent from the year before and the lowest level in 39 years, according to US Customs and Border Protection, a unit of the Department of Homeland Security.

In the previous fiscal year, agents apprehended 447,731 illegal crossers in the Southwest, compared with 1.6 million in 2000, the peak year. The last time the border was this quiet was 1972, when agents caught 321,326 people.

"We have reached the end of an era," said Dowell Myers, a demographer at the University of Southern California. "Even if immigration increases some after this recession, it won't rebound back to levels we saw in the early 2000s."

The dramatic decrease in border arrests -- which the US considers a key gauge of how many people try to enter illegally -- is supported by figures that show a shrinking number of illegal immigrants already in the country.

In 2010, that undocumented population was estimated at 11 million by the independent Pew Hispanic Center, down eight percent from its peak of 12 million in 2007.

At 150,000 last year, Mexican immigration to the US was one-fifth of what it was in 2000, when 750,000 Mexicans flocked to the US, the majority of them illegally.

Nearly 21,500 agents, about twice as many as in 2004, guard the Southwestern border. They are backed by hundreds of miles of fencing and high-tech surveillance, including thermal imaging and unmanned aerial systems.

Mexican drug cartels also may play a role in discouraging people. The cartels often ply the same routes to the US that undocumented immigrants use, making those paths violent and dangerous. Some crossers have been forced to serve as drug carriers for cartels.

Some demographers say more undocumented Mexicans may be leaving the US than arriving as a downturn in construction, hospitality and other industries makes low-skill jobs scarce. Thousands of illegal immigrants have lost their jobs after the US has audited company payrolls to find undocumented workers.

Still, illegal immigration remains a contentious political issue. More than one million people have been deported since President Barack Obama took office in 2009. Deportations hit a record 397,000 in the fiscal year ended Sept. 30. The president favors putting undocumented workers on the path to legalization. But as the 2012 election approaches, no immigration bill is expected to come before the House and Senate.

The impasse has propelled several states, such as Arizona, Alabama and Georgia, to pass laws to curb illegal immigration. Supporters say undocumented workers are taking jobs from Americans at a time of high unemployment and burdening cash-strapped public governments.

Except for former House Speaker Newt Gingrich, who said those in the US more than 20 years should be able to earn legal status, top Republican presidential candidates oppose letting illegal immigrants remain in the US.

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