Illegal immigrants have slipped through America's porous borders at great risk and in great numbers for decades. Estimates put the number of undocumented workers in the country at almost 12 million.

But experts say they are seeing a shift: Illegal immigrants are now choosing to leave the United States. Others are being forced to return to their homeland by aggressive federal law enforcement at the borders and in cities across the country.

The stepped-up campaign comes as immigration advocates ponder strategy following the collapse of reform initiatives in Congress a year ago. Since then, the issue of immigration reform has largely fallen off the national radar. It was barely mentioned during the hotly contested presidential campaign.

That may change soon. With President-elect Barack Obama set to take office in January, advocates are preparing to renew efforts for an immigration package that includes a guest worker program.

Any change to current law may be too late for many undocumented workers who have made the decision to return to their home countries amid a worsening U.S. economic crisis.

They head back to towns and cities across the Western Hemisphere where opportunities for meaningful employment are slim -- and may get even worse.

Studies are showing slippage in the once reliable remittances, the financial linchpin of countless cities and towns in the region.

In this four-part series, The Miami Herald explores the impact of a changing immigration landscape on illegal immigrants, their families and their communities, from South Florida to Latin America and the Caribbean.