Republican presidential hopeful Mitt Romney swept to a big win in his party's primary in Puerto Rico on Sunday, bolstering his position as front-runner in the race to determine who will face Democratic President Barack Obama in the November 6 election.

With 30 percent of the ballots counted, Romney had about 83 percent of the vote, according to Puerto Rico's electoral commission. Rick Santorum was in second place with just over 8 percent. Newt Gingrich was third with about 2 percent.

With more than a simple majority of the vote, Romney looked almost certain to win all 20 of the delegates up for grabs in a contest that focused largely on an upcoming referendum to decide whether Puerto Ricans want to pursue statehood or remain a self-governing U.S. commonwealth.

Puerto Rico Governor Luis Fortuno, a Romney ally, declared victory for Romney early Sunday evening, as official returns trickled in more than three hours after polls closed.

Now is the time to unite in a single Republican Party and send a message to the entire nation that we are with Romney, Fortuno told reporters.

Santorum, a former U.S. senator from Pennsylvania, had posed a potential threat to Romney in Puerto Rico, since his Catholicism and social conservatism were seen resonating among some voters in the predominantly Roman Catholic territory.

But Romney's campaign was endorsed by just about every prominent Republican on the Spanish-speaking island, and Santorum angered many Puerto Ricans with comments last week that they needed to make English their primary language if they wanted to pursue statehood.

You can't impose English on people. My sense is that he (Santorum) was very poorly advised or he would not have said what he said, Ana Lydia Porrata-Doria, 69, who voted for Romney, told Reuters.

Puerto Ricans, who recognize both English and Spanish as their official languages, will vote in November in a statehood referendum.

With Puerto Rico's unemployment rate running at 15.1 percent, many voters said they supported Romney because they believed he was best positioned among the Republican candidates to deliver on pledges about job creation on the island.

Romney may have suffered at least one self-inflicted wound during a visit to Puerto Rico late last week, when he reiterated his opposition to the appointment of Sonia Sotomayor to the U.S. Supreme Court on grounds he did not support her judicial philosophy.

Sotomayor grew up in the fold of the large Puerto Rican community in New York City. Her 2009 appointment to the high court was a source of pride to Puerto Ricans from across the political spectrum.

Puerto Rico, which sits about 1,200 miles from the U.S. mainland, has about 3.8 million people. Its population can vote in partisan primaries but not in presidential elections. Puerto Ricans within the United States have the same voting rights as other U.S. citizens.

Congress would have to give approval for Puerto Rico to become the 51st state. Although U.S. lawmakers have considered various proposals to make English the official U.S. language, none has ever passed.

Romney's win on Sunday was part of a carefully planned island strategy, which has included wins in Guam, the Northern Mariana Islands and U.S. Virgin Islands, to blunt the impact of losses to Santorum in some recent contests.

Romney has a big lead in support from party delegates, whose backing is needed to win the nomination. But he faces a growing challenge from Santorum in Illinois, which holds its primary contest on Tuesday.

The Illinois vote is the next big hurdle in the months-long fight to win the 1,144 delegates needed to seal the Republican nomination.

(Reporting By Reuters in San Juan; Additional reporting by Sam Youngman; Writing by Tom Brown; Editing by Peter Cooney)