WASHINGTON, May 17 (Reuters) - U.S. legislation to overhaul immigration would change the procedure for obtaining permanent U.S. residency, or a green card, and offer a new Z visa for workers who entered the country illegally, according to Bush administration officials.

Leading U.S. senators on Thursday reached an agreement on the legislation, which President George W. Bush endorsed. It must be approved by Congress and signed by Bush to become law.

Following are some facts about the proposed changes.

* A new Z visa would be created for illegal immigrants who can prove they arrived in the United States before Jan. 1, 2007. They would receive a probationary visa while background checks were conducted to ensure they have committed no crime. That would convert to a renewable four-year Z visa allowing employment in the United States. After that renewal, the worker could be eligible for a green card.

* The extent to which family connections get someone a green card would be reduced. Currently about two-thirds of green cards issued are based on family connections.

* People who have applied as of March 2005 for green cards through extended family connections, if qualified, would receive permanent residency within eight years. The United States would increase the number of green cards issued to clear that backlog of about 400,000.

* After the backlog cleared, the system would change. Nuclear families, defined as a spouse or minor children, would still be eligible, but extended family connections would no longer be a basis for preference.

* Most green cards issued going forward would be based on a merit system, with most weight given to education, employment, U.S. work experience, and being in a field, such as health care, where there is a need for workers.

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