LINCOLN, Nebraska — Representative Jeffrey Fortenberry, a Republican congressman from Nebraska, faces growing pressure from congressional leaders and Nebraska’s GOP governor to resign after a California jury found him guilty of lying to federal authorities about an illegal $30,000 campaign donation from a Nigerian billionaire.

Speaker Pelosi and Minority Leader McCarthy both urged the nine-term congressman to leave office, as did the governor of Nebraska, Peter Ricketts, who has endorsed Fortenberry’s top Republican primary challenger.

“The people of Nebraska deserve active, certain representation,” Mr. Ricketts said. “I hope Jeff Fortenberry will do the right thing and resign so his constituents have that certainty while he focuses on his family and other affairs.”

Mr. McCarthy said he texted Fortenberry about the conviction and planned to talk to him about the matter on Friday.

“I think when someone’s convicted, it’s time to resign,” Mr. McCarthy told reporters in Washington, D.C.

Ms. Pelosi said Fortenberry’s conviction “represents a breach of the public trust and confidence in his ability to serve. No one is above the law.”

A federal jury in Los Angeles deliberated for about two hours Thursday before finding Fortenberry guilty of concealing information and two counts of making false statements to authorities. Fortenberry was charged after denying to the FBI that he was aware he had received illicit funds from Gilbert Chagoury. Mr. Chagoury is a Nigerian billionaire of Lebanese descent who lives in Paris.

The three men who funneled money to Fortenberry from Mr. Chagoury were all of Lebanese descent and had ties to In Defense of Christians, a nonprofit Fortenberry supported that was devoted to fighting religious persecution in the Middle East.

Outside the courthouse, Fortenberry said the process had been unfair and that he would appeal immediately. He would not disclose if he would suspend his campaign for re-election, saying he was going to spend time with his family.

“I’m getting so many beautiful messages from people literally all around the world, who’ve been praying for us and pulling for us,” he said.

The judge set sentencing for June 28. Each count carries a potential five-year prison sentence and fines.

Felons are eligible to run for and serve in Congress, but the vast majority choose to resign under threat of expulsion. Congressional rules also bar members from voting on legislation after a felony conviction unless their constituents re-elect them.

It was the first trial of a sitting congressman since Representative Jim Traficant, an Ohio Democrat, was convicted of bribery and other felony charges in 2002.

A campaign spokesman, Chad Kolton, said Friday that Fortenberry had no immediate response to the calls for his resignation.

“He’s spending time with his family right now,” Mr. Kolton said. “That’s what’s most important today.”

Fortenberry’s indictment in October drew a serious Republican primary challenge from Mike Flood, a state senator and former speaker of the Nebraska Legislature. Mr. Flood has since won the endorsements of Mr. Ricketts and a former governor, Dave Heineman. A Flood campaign spokesman declined to comment Friday on the news of Fortenberry’s conviction.

The winner of the GOP primary is likely to face state Senator Patty Pansing Brooks, a Democrat from Lincoln, but Nebraska’s 1st Congressional District is heavily Republican and hasn’t been competitive in decades.

On Friday, Ms. Pansing Brooks said Fortenberry’s conviction was a “wake-up call” that the district needs a change and called for an overhaul of campaign finance laws to increase transparency.

“We cannot allow a return to business as usual as this dust settles,” Ms. Pansing Brooks said, adding that Nebraska is likely “to be the brunt of some jokes” because of Fortenberry’s conviction.

Fortenberry, 61, did not testify at trial but his lawyers argued that he wasn’t aware of the contribution and that agents directed an informant to feed him the information in a 10-minute call to set him up.

An assistant U.S. attorney, Mack Jenkins, said there was ample recorded evidence in the case and the jury’s swift verdict vindicated the prosecution’s efforts.

“Our view is that it was a simple story,” Mr. Jenkins said. “A politician caught up in the cycle of money and power. And like I said, he lost his way.”