GEORGETOWN (Reuters) - Voters in Guyana were heading to the polls on Monday for an election that will decide who oversees an oil boom set to transform the poor South American country, which faces a test of whether it can sustainably manage a sudden influx of natural resource wealth.

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Irfaan Ali, a 39-year-old former housing minister from the opposition People’s Progressive Party (PPP), is seeking to unseat incumbent President David Granger, whose five-year tenure has coincided with the discovery of billions of barrels of crude offshore by a consortium led by Exxon Mobil Corp.

While Ali has not threatened to renegotiate the terms of the country’s contract with Exxon, members of his party have repeatedly slammed the deal signed under Granger - which includes a 2% royalty and 50% profit share after the consortium recovers its costs - as too generous to the U.S. company.

“They sold us out on the oil and gas sector,” Bharrat Jagdeo, a former president and the PPP’s current leader, told a rally of supporters on Saturday. “Next week, when Irfaan Ali is the president, the oil companies will have to come back and talk about a fairer deal for Guyanese.”

Granger, a 74-year-old retired army brigadier, has argued his party would handle oil wealth more transparently, helping Guyana avoid the fate of other petro-states whose governments have wasted oil wealth on graft. His allies frequently bring up corruption scandals involving the PPP, which held the presidency from 1992 to 2015.

“The bulk of the money will go into the sovereign wealth fund, not into people’s pockets,” he said at a closing campaign rally in the capital Georgetown, adding that he would use oil money for infrastructure projects such as a bridge over the wide Essequibo River and a highway through the jungle toward the Brazil border.

While oil is set to provide a significant economic boon in the country of fewer than 800,000, the vote was likely to be marked by the same ethnic divisions that have dominated Guyanese politics since it attained independence from Britain in 1966.

Descendants of Indian laborers who came to work on the sugar plantations largely support the PPP, while the descendants of African slaves largely support Granger’s APNU-AFC coalition.

Granger, who during his term underwent treatment for cancer that he says is now in remission, won by fewer than 5,000 votes in 2015, after taking over parliament in 2011 through a coalition with a smaller multiethnic party that won over many sugar and rice farmers.

But the closure of four sugar estates during his tenure, resulting in substantial job losses for Indo-Guyanese, could send those voters back to the PPP. Ali has said he would welcome back those who “made a mistake in 2015.”

In past elections, it has taken the country three to five days to report official results.

(This story has been refiled to fix typographical error in first paragraph to make it “Guyana” instead of “Guayana”)