At that meeting, Mr. D’Amato said, Adam Skelos made it clear that he was interested in working with or for Park Strategies. But other executives at the lobbying firm, including Mr. D’Amato’s son, Christopher D’Amato, the general counsel for the firm, were “adamant” that Park Strategies not hire the younger Mr. Skelos, Mr. D’Amato said. He said he agreed.

“I thought the appearance would be one that would, in and of itself, raise questions about conflicts,” said Mr. D’Amato, whose firm lobbies members of the State Senate. “Under no circumstances could we bring him in as part of the firm, and we thought it would be one that would just not look — the appearance of impropriety was such that we could not work together.”

Mr. D’Amato has testified at trials before. In 1985, he testified in a case against the Nassau County Republican Party. The next year, he testified on behalf of the United States Football League in its antitrust suit against the National Football League. And in 1988, he was a government witness against Mario Biaggi, a Democratic congressman at the time, and six other defendants for their role in what became known as the Wedtech scandal.

Mr. D’Amato is also personally familiar with the suggestion of wrongdoing: He was the subject of two Senate Ethics Committee inquiries in the 1990s, one that resulted in a rebuke and another in which he was cleared. One of the inquiries grew out of a four-year federal corruption investigation into his younger brother, Armand P. D’Amato, who was convicted of mail fraud in 1993. The conviction was later overturned on the grounds of insufficient evidence, and Armand D’Amato now serves as a partner at Park Strategies.

As Alfonse D’Amato left the courthouse on Friday, he was asked by a reporter how he felt about testifying against his friend. “I didn’t testify against my friend,” he said. “I just answered questions.”

But the forceful yet matter-of-fact testimony from one of the state’s most powerful Republican figures bolstered the testimony of other witnesses, who told the jury that Adam Skelos’s position at P.R.I. amounted to a no-show job, that his behavior on the rare occasions when he did come to work was troublesome and that his father seemed unconcerned. As a result, it could have a powerful impact.

Prosecutors have charged that Senator Skelos and his son monetized the lawmaker’s official position, using his power and influence to direct a real estate developer to funnel a $20,000 payment to Adam Skelos, and to get him the no-show job at P.R.I., as well as consulting work at AbTech, an environmental technology company.