Permanent residents—that is, people who are on a path to citizenship—are covered by basically the same tax laws as citizens. But with no voting rights, they have long been a politically expedient source of funding for congressional spending, said Evelyn Capassakis, a principal at PricewaterhouseCoopers and co-chairwoman of the A.I.C.P.A. task force.

"There's no constituency in the United States that lobbies against this kind of legislation," Capassakis said.

The Heart Act provides tax benefits for veterans, active-duty service members, and reservists. The costs are offset by $411 million in additional expatriate tax revenue expected over 10 years.

The tax applies to all Americans who renounce their citizenship and to foreign workers who relinquish green cards held for at least eight of the past 15 years. The tax applies to unrealized gains above $600,000, but only for people who have an average tax liability of more than $139,000 a year or are worth more than $2 million.

Despite those thresholds, hundreds, if not thousands, of longtime residents will be hit as they try to leave the country, Alden said. And employers, from banks to manufacturers to technology companies, may be required by the terms of existing employment contracts to cover those costs.

In 2007, the National Association of Manufacturers described the proposal to tax expatriates as "potentially devastating" for the industry's many long-term foreign workers. NAM argued that the rules should target people who expatriate to avoid taxes, not workers who return to their home countries for personal reasons and must, by U.S. law, eventually hand back their green cards.

Previously, long-term residents who lost their green cards could avoid taxes on their unrealized gains by spending fewer than 30 days of any year in the United States for 10 years; even then, only U.S. gains were subject to tax.

American companies may respond by sponsoring fewer green cards or filling openings with workers on less-attractive long-term visas, drawing a smaller and potentially less-talented pool of workers to the U.S. One hitch: Spouses of green-card holders can work in the U.S., but foreign spouses of people here on visas in general cannot.

Alternatively, businesses could continue to sponsor workers for permanent residency, on the condition that the employees pay their own expatriation tax if they remain in the U.S. for more than seven years.

Green-card holders now living abroad may consider immediately giving them up under a provision of the Heart Act that allows retroactive dating for nonresidents.

But green-card holders now living in the United States have no way out, lawyers say.

Correction: This article has been amended to accurately describe which foreign workers are subject to this tax.