Only three primaries remain in the battle for the Democratic presidential nomination, and the next one, the biggest, comes in Puerto Rico.

Never mind that the island is not a state and that residents of the commonwealth cannot vote in November's election. They do get their say in the competition between Barack Obama and Hillary Rodham Clinton, both of whom are set to campaign there today.

And its June 1 primary, which polls suggest Clinton is likely to win, offers more delegates - 55 - than 27 states and the District of Columbia.

How come Puerto Rico has more convention clout than such far-from-tiny states as Arkansas, Connecticut, Kansas, Oklahoma and Oregon?

The short answer is that the commonwealth has more people (just under four million) and that the Democratic Party treats Puerto Rico as though it were a state.

For decades, both major parties have allocated at least a few convention delegates to the island and U.S. possessions such as Guam, the U.S. Virgin Islands and American Samoa.

But the Democrats, as part of the national charter they enacted in 1974, mandated that Puerto Rico "shall be treated as a state." The provision was adopted largely in the hopes of wooing Latino voters on the mainland.

The impact of that provision on this year's campaign has been to guarantee that Puerto Rico and its special issues get considerable attention from the candidates. Witness the simultaneous presence of Obama and Clinton this weekend.

"We should use this opportunity wisely to generate from it benefits from Puerto Rico," Roberto Prats, who chairs the party on the island, said in a telephone interview yesterday. "Both candidates already have said they support status determination, meaning that our future should be decided by the people of Puerto Rico."

Obama and Clinton have agreed, if elected, to support a process in which the federal government would let Puerto Rico conduct a binding referendum on whether it should retain commonwealth status or become the 51st state.

Four plebiscites on that question, which included the independence option, have been held since 1967, with voters opting for the status quo each time. No similar vote is planned at the moment.

Clinton, who has done better than her rival among Latinos throughout the primaries, benefits from being more familiar to Puerto Ricans and from representing, as a senator from New York, a substantial Puerto Rican community.

Obama, by comparison, is considered a relative unknown. It doesn't help that his top local supporter, Gov. Anibal Acevedo Vila, was indicted two months ago on charges of fraud and illegal fund-raising in connection with previous elections.

For the national Democratic Party, figuring how to afford Puerto Rico the benefits of being a state is no easy feat. It requires considerable arithmetical contortions.

The national party's formula for determining how many delegates each state gets is based on two factors: its electoral vote and its popular vote for the Democratic presidential candidate in the previous three elections.

Puerto Rico, of course, has no electoral votes. Nor does it cast popular votes for president.

So this year, the Democratic National Committee chose to award the commonwealth a base number of 44 delegates, roughly the same as for states of similar size.

In addition, Puerto Rico, like other places, received a 15 percent add-on - seven delegates - of party leaders and elected officials whose presidential affiliation is determined by the primary.

And it got a four-delegate bonus for holding off on its primary until the final stages of the process.

That adds up to 55 delegates - not counting eight additional unpledged superdelegates.

If those delegates were going to make the difference between victory and defeat for Clinton and Obama, which does not look likely, the situation might be getting a lot more attention.

"It's a reasonable question," said Prats, who is backing Clinton. "But there's no better opportunity than this primary to help educate the American people about Puerto Rico and our issues, especially the status question."

The Republican National Committee, by the way, does not treat Puerto Rico as a state.

At this summer's GOP convention, the commonwealth will have 23 delegates, fewer than South Dakota or Montana, which have far smaller populations. It will have more delegates than only a few states, including Delaware, Vermont and Hawaii.

Contact senior writer Larry Eichel at 215-854-2415 or leichel@phillynews.com.