HOUSTON — In the debate over immigration, they are virtually unheard, unseen: the hundreds of thousands of foreign-born women, many of them in the United States illegally, who toil in America's homes as nannies, cooks and housekeepers, changing diapers and scrubbing floors.

They are jobs of last resort for people whose other options are few.

The lucky ones earn decent wages and build a promising future for their families.

The less fortunate, isolated and apprehensive, suffer a dismaying array of abuses — from exploitively low wages to sexual harassment. Some are forced to sleep in closets; others are threatened with deportation if they complain about overwork.

"These people can be very, very vulnerable, particularly if they're not documented," said Sam Dunning, who oversees social justice programs for the Roman Catholic archdiocese of Galveston-Houston. "If there's any dispute over working conditions, they have very little recourse."

It is, in Dunning's words, a job sector in the shadows — generally excluded from state and federal labor protections.

Experts and activists agree the ranks of household workers are swelling — likely to more than 1 million — although tallying their exact numbers and regulating their workplaces is near-impossible. Employers commonly seek off-the-books arrangements, avoiding contributions toward Social Security or Medicare, and many undocumented women prefer working in the underground economy to minimize chances of deportation.

In one particularly grim case, a wealthy couple went on trial this week on New York's Long Island, on federal charges related to the alleged abuse of two Indonesian women brought to the United States as housekeepers. Prosecutors say the women were held as virtual slaves, beaten and paid no wages except for $100 a month sent to relatives abroad.

In a few cities, activists have begun campaigns to organize domestic workers and raise awareness of their difficulties, but traditional labor tactics — collective bargaining, the threat of striking — are not feasible.

Working conditions were harsh enough to drive Tomasa Compean away from a housekeeping job in Houston that she'd held for 18 years. Over that span her pay edged up from $30 to $50 a day, but her assigned cleaning duties kept increasing, and she felt pressured to work even when sick.

"They treated me poorly," Compean said of the couple who employed her. "They were always asking me to do more and more."