A federal jury found two former associates of New York City Comptroller John Liu guilty Thursday of various counts of campaign-finance fraud, a setback for the comptroller who vowed to continue his bid for mayor amid renewed questions about his campaign's viability.

The jury convicted Xing Wu "Oliver" Pan, a real-estate developer and a fundraiser for Mr. Liu, of conspiring to commit wire fraud as part of a scheme to defraud the city of government matching funds. Mr. Pan's co-defendant in the case, Jia "Jenny" Hou, Mr. Liu's former campaign treasurer, was acquitted of that charge.

Both Mr. Pan and Ms. Hou were found guilty of attempted wire fraud. Ms. Hou was also found guilty of obstruction of justice and making false statements to the Federal Bureau of Investigation. Both face a maximum of 20 years in prison and are scheduled to be sentenced Sept. 20—10 days after the mayoral primary. Their attorneys said they disagreed with the verdict and vowed to appeal.

Mr. Liu, the first Asian-American to be elected to citywide office, was not charged with any crime and has denied any wrongdoing. At the trial, in Manhattan federal court, a prosecutor alleged Mr. Liu had to have known about the unlawful practices in his campaign; Mr. Liu has denied that, and he promised Thursday evening to continue his bid for mayor undeterred.

At a campaign stop in Brooklyn, Mr. Liu said he had "nothing to hide" and he had a "clear conscience." He described Ms. Hou as a "good person" and said he continued to believe in her.

"I'm proud of the way that my campaign has been conducted," he said. "This is an investigation that has been going on now for almost four years, including tapping my phones, interrogating thousands of my supporters. It is what it is. I'm moving forward with my campaign, and at the end of the day the voters of this great city will decide."

Jia 'Jenny' Hou, ex-campaign treasurer for New York City comptroller John Liu, leaves federal court in New York in February. Associated Press

Veteran political observers said that the convictions in the case—plus trial testimony from another longtime aide that she illegally offered to reimburse donors for their contributions—could jeopardize Mr. Liu's ability to get matching funds and become a campaign distraction.

"It's a black mark and a sad day and calls into question, if the comptroller can't police his own campaign operation, how effective is his oversight of taxpayer dollars," said Dick Dadey, executive director of Citizens Union, a civic group.

Costas Panagopoulos, director of the Center for Electoral Politics and Democracy at Fordham University, said the guilty verdicts put Mr. Liu "on the defensive."

"It's unclear how much he will be able to distance himself, given how it may make sense to some people that he was aware of these things," he added.

U.S. Attorney Preet Bharara, who oversaw the case, said in a statement Ms. Hou and Mr. Pan "stuck a knife into the heart" of the city's campaign-finance law as part of a scheme "to corruptly advantage" Mr. Liu's campaign.

"Cases like this give the people of New York yet another reason to be troubled by the electoral process," said Mr. Bharara, who is now prosecuting a string of public-corruption cases against city and state politicians.

When the verdicts were read, Ms. Hou and Mr. Pan showed no emotion and stared straight ahead. They both left the courthouse without commenting. The jury's split decision on the conspiracy count left in question with whom Mr. Pan conspired, the defendants' attorneys said.

Gerald Lefcourt, Ms. Hou's attorney, described the verdict as "unjust" and said his client was not guilty of any crimes. "Unfortunately, she got caught up in the middle of an obsessive quest to get John Liu," he said. Mr. Lefcourt said he believed the government's investigation into Mr. Liu "is still going."

Irwin Rochman, Mr. Pan's attorney, said his client was "collateral damage in the government's war against John Liu."

During their single day of deliberating, the jurors didn't ask for any testimony transcripts or any of the dozens of exhibits produced at the 2½-week trial.

Juror German Cruz of the Bronx was asked by reporters with whom the jury believed Mr. Pan had conspired. "There are plenty of people involved," he said, not explaining further.

Mr. Cruz said Mr. Liu's name hardly came up during their deliberations but he believed "there was a hint that he knew" about the alleged corruption. Explaining why jurors convicted Ms. Hou, Mr. Cruz said, "Because she filed the paperwork."

At trial, the government presented seven witnesses who said they funneled illegal contributions to the Liu campaign at two events, including one Mr. Pan helped organize in what turned out to be a sting conducted by the FBI. They testified that they signed cooperation agreements to avoid possible prosecution.

One witness, Sharon Lee, Mr. Liu's former press secretary and longtime confidante, testified she offered to reimburse donors to her boss's campaign. She was not charged, but she told the jury she knew what she did was wrong. Mr. Liu said he was disappointed in her, but he said she will remain on his campaign.

The verdict raises uncertainty about whether Mr. Liu will receive public matching funds from the city's Campaign Finance Board. Without the $3 million in public funds he is hoping to get, Mr. Liu would be hamstrung in his ability to hire staff, run advertisements and otherwise mount a vigorous campaign.

To be eligible for matching funds, campaigns have to show that they have complied with the law. The board will determine in the coming months which candidates "have qualified for payment, considering all the information available," a board spokesman said.

—Andrew Grossman, Chad Bray and Joe Jackson contributed to this article.

Write to Sean Gardiner at sean.gardiner@wsj.com and Michael Howard Saul at michael.saul@wsj.com