SAN JUAN, Puerto Rico (CNN) -- A heavy turnout was expected
                  Sunday as Puerto Ricans headed for the polls in a non-binding referendum
                  on whether to petition the United States for statehood.

                  The Caribbean island has been a possession of its North American
                  neighbor for 100 years.

                 The vote is Puerto Rico's third plebiscite on the issue since 1967,
                 when retaining the island's commonwealth status won over a
                 proposal for statehood by a nearly 2 to 1 margin. Five years ago the
                 margin was much closer, with 48.6 percent of voters opting for the
                 commonwealth while 46.3 percent preferred statehood.

                   "We have seen over the years a decrease in support for
                   commonwealth status," Governor Pedro Rossello, who supports
                   statehood, said as he voted Sunday morning.

                  Polls show close contest
 
                  Rossello earlier predicted a victory for statehood, despite polls that showed
                  the contest too close to call.

                  The choices put before the voters include: push for statehood, retain
                  commonwealth status, begin a "free association" with power for local
                  government and close ties to the United States, seek independence, or
                  none of the above.

                  Backers of commonwealth status, arguing that the ballot was worded to
                  favor statehood, have urged supporters to vote "none of the above."

                 "We're very happy with our commonwealth, and we want to
                  keep it," said San Juan Mayor Sila Calderon as she voted. "Today, we
                 Puerto Rican people are asserting our identity, our love for
                 commonwealth, our love of our U.S. citizenship."

                  Puerto Rico, a Spanish colony for 400 years, became a U.S. territory
                  at the close of the Spanish-American War in 1898, and took on its current
                  commonwealth administration in 1952. Under it, Puerto Ricans are
                  considered U.S. citizens but have no voice in U.S. government and
                  are not required to pay U.S. taxes.

                 Eighty percent turnout predicted

                 Voting began early Sunday, and polls close at 3 p.m. (1900 GMT).
                 Initial results are expected after 6 p.m. (2200 GMT). Election
                 officials predicted that as many as 80 percent of Puerto Rico's 2.2
                 million registered voters would cast their ballots by day's end.

                 "We will be protecting the future of our children," said 72-year-old
                  Ernesto Cabrera, who said he voted for statehood.

                  "It's very important because today is the day that could decide what
                  Puerto Rico desires," said 40-year-old pharmacy technician Juan
                  Aponte. "We have to demonstrate to the federal government what
                  the people want."

                  Gov. Rossello called for the referendum in July, hoping to persuade the
                  United States to consider making Puerto Rico the 51st state.

                  "I'm confident that statehood would maintain the majority of votes," he
                  said before the referendum. "This is a historical movement. It has been going
                  on since the establishment of the commonwealth."

                  But some analysts predicted that nothing short of a landslide for
                  statehood would sway the U.S. Congress.

                  "If Puerto Rico votes less than half, or only 51 or 52 percent in favor of
                  statehood, the United States government is not going to let Puerto Rico
                  become a state," said Thomas Boswell, a professor of geography in the
                  School of International Studies at the University of Miami.

                       The Associated Press and Reuters contributed to this report.