FACTBOX - Puerto Rico and its presidential primary

(Reuters) - Democrats Hillary Clinton and Barack Obama face off in the U.S. territory of Puerto Rico on Sunday in the state-by-state battle to determine who will face Republican John McCain in the November U.S. presidential election.

Following are some facts about Puerto Rico:

* Residents of this Spanish-speaking Caribbean island cannot vote in the presidential election but can participate in the primary process. Puerto Rico has 55 delegates at stake in its contest, more than 28 U.S. states.

* Puerto Rico's relationship with the United States is the central issue in the island's politics. Both Clinton and Obama support allowing Puerto Ricans to decide for themselves whether they want to try for statehood or keep their current status.

* Median household income in Puerto Rico is $17,621 (8,900 pounds), roughly half that of the poorest U.S. state. While 3.9 million Puerto Ricans live on the island, a similar number live on the U.S. mainland.

* Voting ends at 3 p.m. EDT (8 p.m. British time) with results expected shortly after. Clinton, who has drawn strong support among Hispanics, is expected to do well in Puerto Rico. Both candidates' spouses have campaigned on the island.

SOURCES: U.S. Census Bureau; Almanac of American Politics; Democratic National Committee; CNN; Puerto Rico Board of Elections

(Compiled by Andy Sullivan; editing by Mohammad Zargham)

(To read more about the U.S. political campaign, visit Reuters "Tales from the Trail: 2008" online at blogs.reuters.com/trail08/)