NEWARK, N.J. — When a wealthy donor to Sen. Bob Menendez asked government officials for help in a contract dispute and was rebuffed, the donor’s representative responded with bluster and threats, claiming he had political connections that could “cause a lot of problems,” a U.S. Commerce Department official testified Monday.

The official, Scott Smith, told jurors he took the comment to refer to Menendez.

The interaction touches on the government’s central thesis in the bribery trial of the New Jersey Democrat and Florida eye doctor Salomon Melgen: that Menendez used his influence to lobby on behalf of Melgen in exchange for flights on Melgen’s private jet and visits to his villa in an exclusive resort in the Dominican Republic.

On Monday, the beginning of the trial’s fourth week, prosecutors began focusing on ICSSI, a company Melgen bought into in 2011 that provided X-ray screening equipment for Dominican ports. A change in government had brought with it a dispute over the contract, and ICSSI had been at loggerheads with the Dominican government for several years when Melgen came on board.

Melgen’s solution, prosecutors allege, was to have Menendez, his longtime friend, push for the U.S. government to force the Dominican government to honor the agreement and also rescind a donation of U.S. screening equipment to the Dominicans because it would hurt ICSSI’s business.

A former State Department official in the Dominican Republic testified Monday he was told by superiors in 2011 not to intercede. In 2012, an attorney for Melgen approached the Commerce Department with a similar request.

Attorney Elio Muller “was very aggressive and threatening” and said he could be “a bull in the Commerce Department china shop,” Smith testified. Muller said Melgen “had politically influential friends and they could cause a lot of problems” if Melgen felt his interests weren’t being served, Smith added.

“Who did you understand this to mean?” Justice Department attorney Monique Abrishami asked.

“Senator Menendez,” Smith replied.

The indictment alleges Menendez met with an assistant secretary of state regarding the matter in May 2012, on the same day Melgen and his family gave $40,000 to a state Democratic committee campaign fund and $20,000 to a legal defense fund set up by Menendez.

Earlier Monday, defense attorneys tried to undermine the government’s narrative by showing Menendez took numerous flights to the Dominican Republic to visit Melgen that he paid for himself during the time the alleged bribery scheme was underway.

Prosecutors have shown that Menendez didn’t report the free flights, or a $1,500-per-night hotel stay in Paris in 2010 paid for by Melgen’s American Express points, on his Senate disclosure forms. Menendez later said it was an oversight and reimbursed Melgen for about $58,500.

On Monday, FBI Agent Alan Mohl echoed earlier testimony that Menendez didn’t reimburse Melgen for some of the flights until early 2013, after reporters began asking questions about the two men’s relationship.

The defense contends the indictment criminalizes the pair’s friendship by drawing parallels between trips Menendez took, which it says were part and parcel of his friendship with Melgen, and meetings and other interactions he had with government officials, which it says were within the scope of his duties as a senator.

The trial is expected to last several more weeks.