Nicolás Maduro was declared the winner in Venezuela’s presidential election Sunday, even as his opponents were preparing to dispute the results, setting up a high-stakes showdown that will determine whether the South American nation transitions away from one-party rule.

Shortly after midnight, the National Electoral Council said Maduro secured 51% of the vote, overcoming opposition candidate Edmundo González, who garnered 44%. It said the results were based on a tally of 80% of voting stations, marking an irreversible trend.