The start of oil production in December is expected to nearly double the country’s gross domestic product in 2020, according to the International Monetary Fund, and multiply in years to come.

Instead, the winner-takes-all attitude that has marred the elections is weighing heavily on Guyana’s economic prospects as it enters the oil age, said Ralph Ramkarran, a prominent local statesman who led a largely Quixotic campaign for a small multiethnic party.

“The thinking here is, ‘why share when you’re winning?’” he said. “Until that’s fixed, it will remain a place of suspicion and economic underdevelopment.”

The stakes could not be higher.

Exxon started production in December, and although the payoff in 2020 will be a trickle relative to what will come, it is expected to elevate oil income this year to a third of all government revenue, surpassing all of the country’s traditional exports combined, according to the I.M.F.

By the end of the decade, the country’s output will reach 1.2 million barrels a day, according to estimates by the oil consultancy Rystad. That would mean Guyana’s production would overtake the current output of its neighbor, the declining oil giant Venezuela.

The economic decisions taken by the next government will largely determine whether the former British sugar-growing colony is able to harness its oil wealth for national development. But neither major party has offered a plan for the nation.