He walked jurors through the timing of a donation by Dr. Melgen and communications between Mr. Menendez and officials at the Department of State. It began, he said, with an email on April 30, 2012, from Mr. Menendez’s chief of staff to Dr. Melgen asking for $60,000. On May 16, the doctor made a donation to groups supporting Mr. Menendez, the same day that Mr. Menendez met with an assistant secretary of state to discuss port security in the Dominican Republic, an area in which Dr. Melgen had a financial interest.

He offered similar summaries of timing and action involving a meeting among Mr. Menendez and officials from the federal Centers for Medicare and Medicaid Services, as well as a later meeting with Kathleen Sebelius, then the secretary of Department of Health and Human Services.

Mr. Koski also noted repeatedly that Dr. Melgen was not a constituent of Mr. Menendez, telling jurors that Mr. Menendez’s website said, “If you are not a New Jersey resident, I will refer your inquiry to your home state senators.”

He continued: “All this to enrich one man who didn’t even live in the state Mr. Menendez was elected to represent.”

But Mr. Lowell argued that senators are “national legislators” who have interests and obligations outside their state.

And he sought to rebut the corrupt behavior that the prosecution was alleging. The meeting with health agencies, Mr. Lowell said, was to intervene in what Mr. Menendez considered overbilling by big pharmaceutical companies in placing limits on the use of eye medicine and was not solely about a billing dispute involving Dr. Melgen as the prosecution described.

Mr. Lowell continued: As for the port in the Dominican Republic, Mr. Menendez was motivated not by Dr. Melgen’s interests but more broadly by national security concerns. And when Mr. Menendez helped secure visas for Dr. Melgen’s girlfriends, he was simply doing something he had done thousands of times as a senator.