WASHINGTON (Reuters) -U.S. President Donald Trump is expected to attend the opening on Tuesday of a temporary migrant detention center in southern Florida dubbed "Alligator Alcatraz", a source familiar with the matter said. The step comes as Trump, a Republican, has sought to ramp up the detention and deportation of migrants, saying the measure was needed after millions crossed the border illegally under Democrat Joe Biden. The center got the nickname from its remote location in the Everglades, a vast subtropical wetland teeming with alligators, crocodiles and pythons that a Florida official said this month provides natural barriers, requiring minimal security.