(Christopher Gregory/AP Photo)

Mitt Romney has been declared the winner of Puerto Rico's primary.

Romney and Rick Santorum both campaigned in the commonwealth in the days leading up to the primary, and both came to this contest with their own weapons in their arsenal.

Romney had the endorsement of Puerto Rico Gov. Luis Fortuno, who endorsed Romney in January, just days before the Florida primary.

Santorum touted his history of support for the island when he served in the Senate, telling reporters that he "was referred by many in my state as Senadore Puertorriqueno" because of his vocal support for the territory. Santorum generated controversy, however, when he said he believes Puerto Rico must adopt English as its "main language" in order to be recognized as state.

Romney thanked Fortuno in a paper statement on Sunday night.

"Ann and I are deeply grateful to Governor Fortuno, his wife Luce Vela, and the good people of Puerto Rico for their support and help in winning the Republican primary" the statement read.

Puerto Rico's primary marks the sixth island victory for Romney. He also won the voting contests in Hawaii, Guam, the Virgin Islands, the Northern Mariana Islands and American Samoa.

Republican Party officials in Puerto Rico tell ABC News that primary turnout was high, and that they were very happy with the attention the GOP candidates gave to the contest.

ABC News projects that Romney will win all 20 delegates awarded in Puerto Rico's contest, bringing him one step closer to the 1,144 delegates needed to secure the GOP nomination before the convention. Romney currently has 521 delegates. Rick Santorum has 253, Gingrich has 136, and Ron Paul has 50.

ABC's Emily Friedman contributed to this report.