A federal jury convicted former Representative David Rivera of Florida on Friday, finding him guilty of conspiracy and six other crimes for secretly lobbying officials in Washington on behalf of the Venezuelan government in 2017 and 2018.

Prosecutors presented evidence during the five-week trial in Miami showing that Venezuela’s state-run oil company had secretly hired Mr. Rivera’s consulting firm for $50 million to lobby members of Congress and the White House for a thaw in U.S.-Venezuela relations.

The revelation ran contrary to how Mr. Rivera, a Republican, had portrayed himself in public. He made a political career, first as a state lawmaker and later as a congressman, as a strident anti-Communist. Mr. Rivera served in Congress from 2011 to 2013.

He had previously been the subject of several state and federal investigations into improper campaign dealings. He was also found guilty in the criminal case of failing to register as a foreign agent and money laundering, and faces about 10 years in prison.

His defense lawyers in the criminal case had argued that Mr. Rivera was not working for Nicolás Maduro’s government but rather surreptitiously trying to oust him. They also said that Mr. Rivera did not need to register as a foreign agent because his firm’s contract was with an American company, PDV USA, a U.S. subsidiary of the Venezuelan state-run oil company, Petróleos de Venezuela, and not with the state-run company itself.

The 12-member jury also convicted one of Mr. Rivera’s associates, Esther Nuhfer, on four charges. Prosecutors said that Mr. Rivera, 60, split the secret contract earnings, which ultimately amounted to about $20 million after the company terminated the contract, with Ms. Nuhfer and two people who were not charged in the case. Ms. Nuhfer, 52, is a political consultant based in Miami.

Roger Cruz, an assistant U.S. attorney and the lead prosecutor, said in his closing argument on Tuesday that Mr. Rivera and Ms. Nuhfer decided to keep the contract secret because of “greed.”

“Without their keeping it secret, they would not have got a single penny,” he said. “If anyone found out, their careers would be over.”

The trial drew widespread attention when it began because prosecutors called Secretary of State Marco Rubio to testify against Mr. Rivera, his longtime friend and former housemate in Tallahassee when they both served in the Florida Legislature.

Mr. Rubio, who has not been implicated in any wrongdoing, was a Republican U.S. senator from Florida in the years that Mr. Rivera was secretly lobbying for Venezuela. Mr. Rubio held two meetings with Mr. Rivera at that time and testified in court that he had no idea about Mr. Rivera’s secret contract.

Other prosecution witnesses included Brian Ballard, a major lobbyist and top fund-raiser for President Trump, and Hugo Perera, one of the other two men who admitted to taking part in the conspiracy. Mr. Perera was not charged because he agreed to testify against Mr. Rivera and Ms. Nuhfer.

Mr. Perera testified that Mr. Rivera and Ms. Nuhfer had kept the contract secret because they knew it would create a political scandal if it became public. Defense lawyers noted that Mr. Perera, a developer who had served prison time for cocaine trafficking and tax fraud in the 1990s, was allowed to keep the roughly $5 million he made from the Venezuela deal.

One of the defense witnesses was Representative Pete Sessions of Texas, a Republican, who testified that he worked with Mr. Rivera in 2017 to try to persuade Mr. Maduro to step down and hold presidential elections. Mr. Sessions also said that he did not know at the time about Mr. Rivera’s secret Venezuela contract.

Edward R. Shohat, one of Mr. Rivera’s defense lawyers, told jurors in his closing argument that prosecutors had tried to confuse them. “All that he was about was removing Mr. Maduro,” Mr. Shohat said of Mr. Rivera.

David O. Markus, a defense lawyer for Ms. Nuhfer, said she had signed onto the contract “in good faith,” believing it was with a U.S. subsidiary. She would never “in a billion years” have tried to help the Maduro government, Mr. Markus said.

Senator Chuck Schumer, the Democratic minority leader, and Graham Platner, the party’s now presumed candidate in Maine’s crucial Senate race, talked by telephone on Thursday after Gov. Janet Mills, Mr. Schumer’s favored candidate, abruptly suspended her campaign.

A person familiar with the call would provide few details when asked about any contact between the two men, but acknowledged they had spoken and said they agreed that Democrats needed to win Maine and take back the Senate majority from Republicans. Another person familiar with the conversation said it was cordial and stressed the importance of unseating Senator Susan Collins, the five-term Republican incumbent.

It was the first exchange between them. Mr. Schumer, the New York Democrat long deeply involved in his party’s Senate campaign strategy, had heavily recruited Governor Mills and saw her as the party’s best opportunity to defeat Ms. Collins, the chairwoman of the Appropriations Committee who has repeatedly fended off Democratic challengers.

But Mr. Platner, a populist oysterman and political novice, proved formidable despite Governor Mills’s political pedigree. Mr. Schumer now finds himself trying to establish a relationship with the party’s presumed candidate in what is likely a must-win race if Democrats are to have any chance of taking the Senate.

The decision by Governor Mills was a significant setback for Mr. Schumer that underscored discontent about him among some in the party and had some Democrats questioning whether he had lost a step in his political calculations. But he struck an optimistic tone as he left the Capitol on Thursday night.

“We are going to beat Susan Collins and we are going to win back the Senate,” he told reporters.

Democrats believe that voter discontent in Maine with the direction of the Trump administration gives them a strong chance to defeat Ms. Collins, who has proved politically resilient at home.

Annie Karni contributed reporting.