Explainer-What is happening with Venezuela's election?

Venezuelan presidential candidate and governor of Zulia, Manuel Rosales, addresses the media, in Caracas

CARACAS (Reuters) - A presidential election is set to take place this year in Venezuela for the first time since 2018, but the opposition is wrestling to define a unity candidate who can take on President Nicolas Maduro, after numerous possible candidates were either barred from office or prevented from registering.

WHEN IS THE ELECTION?

Venezuela's presidential election is set to take place on July 28. It is a simple majority-wins vote.

WHO ARE THE CANDIDATES?

President Nicolas Maduro of the Socialist Party, who has been in power since 2013, has said he will run for another six-year-term. His government is under U.S. sanctions and has presided over a sharp economic and social deterioration.

Two opposition groups have registered contenders, but it is unclear if both will remain on the ballot or whether they will be able to rally significant support.

Ten other candidates, who have minimal support, have also registered to run but are seen as government allies by the opposition.

WHO ARE THE POSSIBLE OPPOSITION CANDIDATES?

The opposition held a multi-party 2023 primary to select one candidate to face Maduro.

The opposition's right to choose its own candidate was included in an election deal inked with the government in October, an accord which prompted the United States to ease oil sanctions.

Many well-known opposition figures, including former presidential candidate Henrique Capriles, have long been banned from holding office by the country's controller general, for reasons the opposition say are invalid. Capriles dropped out ahead of the primary last year.

Maria Corina Machado, a 56-year-old industrial engineer who supports privatizing state oil company PDVSA, won the October primary by a landslide, notching 93% of the vote, despite being under a ban.

Though she appealed the measure, the Supreme Court upheld it in January on her support of sanctions and accusations of corruption, which Machado denies.

In response, the U.S. reimposed some sanctions and said restrictions on oil will come back into force in April unless she is allowed to run.

Machado continued campaigning, despite the arrests of multiple allies and activists, which sources close to the ruling party said were likely a government reaction to declining support for Maduro.

Some in the opposition had put pressure on Machado to name an alternate candidate, and she eventually named academic Corina Yoris.

But Yoris was unable to register on the electoral council's online system by a March 25 deadline.

WHO WAS ABLE TO REGISTER?

Two opposition groups that were part of the broad coalition backing Machado registered other candidates just before the deadline.

The Democratic Unity group listed Edmundo Gonzalez, a former ambassador, but sources told Reuters his registration was meant as a placeholder for a potential substitute.

Meanwhile, the A New Time party registered Manuel Rosales, the governor of Zulia province.

Rosales, who some in the opposition say has become too close to the ruling party during his governorship, said he registered to ensure there was an opposition candidate and prevent absenteeism.

WHAT HAPPENS NEXT?

Substitute candidates can be named until April 20, so the opposition has several weeks to hold internal negotiations about whether to back Rosales or Gonzalez or substitute others.

Asked this week if she would consider an alliance with Rosales, Machado said she is taking decisions "day by day". She earlier insisted the coalition's candidate is still Yoris.

Rosales said on Tuesday he is willing to cede his place to a unity candidate if the opposition can choose one.

Some in the opposition have said they fear any candidate or substitute could be subject to a public office ban before the July 28 contest.

WHAT DOES POLLING PREDICT?

Various opinion polls had forecast Machado would pick up at least 50% of votes, while Maduro, a 61-year-old former bus driver, was projected to receive around 20%.

It was unclear if any other opposition figure could rally as much support as Machado, especially if she does not wholeheartedly back them.

(Writing by Julia Symmes Cobb, Editing by Rosalba O'Brien)