FACTBOX-Puerto Rico and its presidential primary

(Reuters) - Democrats Hillary Clinton and Barack Obama face off in Puerto Rico on June 1 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 November presidential election but can participate in the state-by-state 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, roughly half that of the poorest U.S. state. While 3.9 million Puerto Ricans live on the island, roughly that same 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 here. 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)

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