The legislation was first presented in November 2012 by Spain’s foreign and justice ministers as a conciliatory gesture toward Sephardic Jews, whose ancestors were expelled in 1492 in one of the darkest chapters in Spanish history.

Leon Amiras, chairman of an association for immigrants to Israel from Latin America, Spain and Portugal, said that he planned to apply for Spanish citizenship and that some families had books or documents tracing and proving their ancestry. When his own grandmother and great-grandmother left Izmir, Turkey, for Argentina, they were issued an identity document signed by Jewish community leaders and certified by the Spanish consul there at the time.

Mordechai Ben-Abir, 88, Mr. Amiras’s uncle, said he hoped to be the first to obtain Spanish citizenship if the law was passed. Mr. Ben-Abir, who lives in the southern Israeli city of Beersheba, began researching his family roots when he was in his 70s, went on to obtain a doctorate in philology at the University of Barcelona in Spain and has traced his Catalan ancestry back to the expulsion of 1492. To return to Spain more than 500 years later with a Spanish passport, he said, would be “a victory” for his family and the Jewish people.

The Justice Ministry of Spain said that it had no estimate of how many Sephardic Jews might be eligible for Spanish citizenship. So far, the ministry has registered 3,000 applications, but a spokeswoman said that number was expected to increase.

Rachel Delia Benaim, an American student living in New York who has Sephardic ancestry, said by email that being allowed to keep her United States citizenship made the Spanish offer “a lot more appealing.” But she remained wary about how certification would ultimately be granted by Spain and said, “Any excitement about the legislation is premature.”