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Puerto Ricans will vote Sunday in the Republican presidential primary, but that may not be the most important one they cast this year - in November, the thorny issue of statehood will come to a referendum.

The island commonwealth has long debated the question, a sensitive one for many there. If Puerto Rico does vote to become the country's 51st state, Congress would then have to approve the move before it could take effect. In the past, Congress has mulled over measures of Puerto Rican statehood, but none have passed.

This year the leading Republican candidates have all said they would work with Puerto Rico to become a state if that is what its citizens decide. Some Puerto Ricans favor becoming a state, while others oppose the idea, preferring to remain a commonwealth or to become independent. In January, Mitt Romney and Newt Gingrich were confronted with the issue at an event in Miami only days before the Florida primary.

"I expect the people of Puerto Rico will decide that they want to become a state and I can tell you that I will work with [Puerto Rico Gov. Luis Fortuno] to make sure that if that vote comes out in favor of statehood that we will go through the process in Washington to provide statehood to Puerto Rico," Romney said.

Gingrich, for his part, said that "if the people of Puerto Rico make the decision that they want to be a state, I will work actively to help them negotiate the process of accession to the United States, but the people of Puerto Rico have to decide their future."

However, when Rick Santorum weighed in on the issue this week in a campaign swing through Puerto Rico, the former Pennsylvania senator sparked controversy by introducing an important caveat: the island territory must adopt English as its main language. Puerto Rico currently lists both English and Spanish as its official languages, but the latter is clearly the dominant one there. Adopting English, Santorum said, should be "a condition" if the island becomes a state.

"In order to fully integrate into American society, English has to be a language that is spoken here also and spoken universally," Santorum said Thursday.

His comments sparked a backlash, even causing one of his delegates - Oreste Ramos, a former Puerto Rico senator - to withdraw his support. Ramos said that "such a requirement would be unconstitutional, and also would clash with our sociological and linguistic reality," therefore he "cannot back a person who holds that position."

The Constitution does not denote any language requirement for statehood. However, no state has ever been admitted into the union without having English as its main language. Puerto Rico's Gov. Luis Fortuno, who has endorsed Romney in the primary, said Friday that he differs with Santorum on the issue of the English requirement, stating that the language question "should not be politicized."

"Since the very beginning of the 20th century, English and Spanish have both been official languages in Puerto Rico. We're proud of that," Fortuno said in an interview with ABC News. "I don't think language should be used in any partisan context or certainly regarding status."

Fortuno noted that he believes that Puerto Rico will elect to become a state, pointing out the quirk that the island's voters can currently vote in the GOP primary, but not in the general election.

"Puerto Rican Americans have fought in every single war with valor and courage since 1917 in greater numbers than most states," Fortuno said. "In the most recent War on Terror, we have contributed more men and women to that war that every single state but one. … The question that everyone in America should ask themselves is, if these American men and women having actually contributed in so many ways, including with their lives to our nation. Don't they deserve to express themselves and tell us whether they want to remain a territory or become an independent republic or become a state with all the obligations and benefits of American citizens residing elsewhere?"

"What the irony of this primary is - it is not just that we will voting here and then we won't be able to vote in November. The greatest irony of all is that we have sent hundreds and hundreds, perhaps thousands, of absentee ballots to Afghanistan and Iraq to men and women in uniform that will vote in the primary but will not be able to vote for the commander in chief."

That irony is one that already reared its head at a debate in January in Florida. Elizabeth Cuevas-Neunder, the CEO of the Puerto Rican Chamber of Commerce in Florida, asked the GOP candidates about the issue of statehood at a CNN debate, but the question went unanswered by all but Santorum before moderator Wolf Blitzer moved on to other topics - and even Santorum was told to deliver a "brief response." The next day, at the Hispanic Leadership Network conference in Miami, Cuevas-Neunder voiced displeasure about that fact, citing that "our Puerto Ricans have given more men and women to the United States Armed Forces than any other state in the union."

It is a bizarre type of political limbo: the ability to serve in the armed forces and vote in presidential primaries, but not general elections. That could all change in November if the island commonwealth elects to become a state, something that each day appears more and more likely.

Matthew Jaffe is covering the 2012 campaign for ABC News and Univision.