Like tens of thousands of Cubans, Gerardo Luis wants to get to the United States and he's suddenly worried that time may be running out.

Across an island where migrating north is an obsession, the widespread jubilation over last week's historic U.S-Cuba detente is soured by fear that warming relations will eventually end Cubans' unique fast track to legal American residency.

For nearly a half-century, the Cuban Adjustment Act has given Cubans who arrive in the U.S. a virtually guaranteed path to legal residency and eventual citizenship. The knowledge that they will be shielded from deportation has drawn hundreds of thousands of Cubans on perilous raft trips to Florida and land journeys through Central America and Mexico.

"If they take away the adjustment law, it would mean Cubans would end up just like all the other Hispanics who want to enter the United States," said Luis, a 36-year-old construction worker who said he may try to reach Mexico and walk across the border if he doesn't get a visa soon.

U.S. officials say there are no immediate plans to change immigration laws or policy. But with the U.S. and Cuba negotiating a return to full diplomatic relations, many Cubans are wondering how long their extraordinary privilege can survive under restored diplomacy, and are thinking about speeding up plans to get to the U.S.

"I don't know if they will take it away," Angela Moreno, a 67-year-old retiree said of the preferential treatment, "but if they do, Cubans who go to the United States will have to do it like people from other countries."

Cubans arriving at a U.S. border or airport automatically receive permission to stay in the United States under policies stemming from the 1966 act, which allows them to apply for permanent residency after a year, almost always successfully.

Seeking to discourage mass migrations by sea, the United States developed its so-called "wet foot, dry foot policy," in which migrants who make it to the U.S. are automatically allowed to stay. Those stopped at sea are either sent back to their homeland or to a third country if they can prove a credible fear of persecution.

U.S. Sen. Bill Nelson, a Florida Democrat, said he welcomed President Barack Obama's move to create a "modern relationship" with Cuba, but Congress is not likely to alter the Cuban Adjustment Act or the U.S. trade embargo, until there have been significant steps by the Castro government.

"Major changes to a law like that or to the embargo are not going to happen unless people like me support those changes, and I'm not going to support them unless I see some movement toward freedom" Nelson told The Associated Press.

However, the restoration of diplomatic relations could cause its own complications.

Illegal immigrants caught right after crossing the border are subject to swift deportation without a hearing, a process known as expedited removal. Cubans are exempted simply by presenting proof of their nationality.

Randy McGrorty, the director of Miami's Catholic Legal Services, which helps migrants settle in the United States, noted that a section of the Immigration and Naturalization Act dealing with expedited removal of migrants excludes people from "a country in the Western Hemisphere with whose government the United States does not have full diplomatic relations" without mentioning Cuba by name. It's unclear how re-establishing full relations would affect that vital section of immigration law, he said.

The Cuban Adjustment Act was designed in an era when politics was a factor in many migrants' decision to leave. In recent years Cubans have increasingly left primarily to reunite with their families and seek better economic opportunities.

President Raul Castro's 2012 relaxation of Cuba's immigration laws means citizens can leave without applying for special permits, and maintain many of the island's social benefits even after moving to the U.S. Along with U.S. measures including the granting of five-year, multiple-entry visas, that has contributed to a rising flow of Cubans and Cuban-American traveling back and forth between the two countries.

U.S. Sen. Marco Rubio, a Cuban-American Republican, has criticized a growing tendency by migrants to constantly travel between the island and the United States, warning that it endangers the special treatment Cubans have long enjoyed.

If the preferential policy is in place to aid exiles fleeing political oppression, such travel "undermines that argument," Rubio said in August. "That sort of travel puts at risk the status Cubans have."

The U.S. Interests Section, which handles American consular affairs in Havana in the absence of a full embassy, approved more than 33,000 non-immigrant visas for Cubans to visit the U.S. last year, a 99 percent increase.

It has also become must faster to get such a visa, with the wait time dropping from 57 months in 2012 to five months this year. Still, it can be a grueling process. Cubans spend hours each day, starting at dawn, in a plaza near the U.S. mission that is known as the "park of sorrows" because of the long waits and the frequent rejection of paperwork.

One waiting outside the interests section this week was 75-year-old Magaly Ruedas, who wants to join her extended family in the United States and hopes the benefits for Cubans will remain in place. If the law changed, "I think that would hurt Cubans and I don't think Obama or Raul intended to do that," she said.

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