And in case the message was still not clear, Milton L. Williams Jr., a lawyer for a third defendant, said Mr. Howe was “a liar who is trying to save his own neck.”

Mr. Howe’s testimony has been eagerly anticipated ever since federal prosecutors announced in 2016 that he had pleaded guilty and was cooperating with authorities in an investigation of Joseph Percoco, a former senior aide to Gov. Andrew M. Cuomo and one of his closest friends. (Mr. Cuomo has not been accused of any wrongdoing.)

The government has charged that Mr. Percoco, one of four men on trial, accepted more than $300,000 in bribes from Mr. Howe’s clients in return for taking official actions on their behalf.

Mr. Howe, who had lobbied in Albany and Washington, has longstanding ties to the Cuomo family. He worked in the administration of former Gov. Mario M. Cuomo, the current governor’s father, and he also worked under Andrew Cuomo when he served as secretary of the Department of Housing and Urban Development under President Bill Clinton.

Defense lawyers’ attacks on Mr. Howe in Mr. Percoco’s trial began early and relentlessly, as they warned the jury that Mr. Howe could not be believed — a common defense tactic to try to weaken the credibility of a key witness.