When Lev Parnas was arrested in 2019, he was known mainly as a minor presence in Republican political circles: someone who traded on connections with allies of Donald J. Trump, including the president’s personal lawyer, Rudolph W. Giuliani, to gain access to other Republican candidates.

But prosecutors say Mr. Parnas had broader — and illegal — aims: He conspired to funnel money from a Russian oligarch to candidates as part of an influence-buying scheme to benefit a cannabis business, according to charges that led to his conviction last year on campaign finance offenses.

On Wednesday Judge J. Paul Oetken of Federal District Court in Manhattan sentenced Mr. Parnas to 20 months in prison.

“Mr. Parnas was at the center of three criminal schemes,” Judge Oetken said, adding that Mr. Parnas had behaved in ways that “erode confidence” in the electoral system.

Just before being sentenced, Mr. Parnas sobbed and apologized, saying he would repay people he had stolen from and would be an “upstanding human being.”

“I have made mistakes, I lied,” he said. “I’m going to be a different person.”

After his arrest, other details of Mr. Parnas’s life emerged. He acknowledged participating in an effort by Mr. Giuliani to pressure Ukrainian officials to investigate President Biden while he was a leading Democratic presidential candidate. And he admitted to conspiring to defraud investors in an anti-fraud company he created called Fraud Guarantee.

In a statement, Damian Williams, the United States attorney for the Southern District of New York, said: “Not content to defraud investors in his business, Fraud Guarantee, out of more than $2 million dollars, Parnas also defrauded the American public by pumping Russian money into U.S. elections and lying about the source of funds for political contributions.”

Mr. Parnas’s lawyers had requested that he be sentenced to time served, writing to Judge Oetken that their client had already spent over nine months under 24-hour home confinement and another 18 months under curfew.