After the inauguration, both Mr. Zamel and Mr. Nader visited the White House, meeting with Mr. Kushner and Mr. Bannon.

At that time, Mr. Nader was promoting a plan to use private contractors to carry out economic sabotage against Iran that, he hoped, might coerce it to permanently abandon its nuclear program. The plan included efforts to deter Western companies from investing in Iran, and operations to sow mistrust among Iranian officials. He advocated the project, which he estimated would cost about $300 million, to American, Emirati and Saudi officials.

Last spring, Mr. Nader traveled to Riyadh for meetings with senior Saudi military and intelligence officials to pitch his Iran sabotage plan. He was convinced, according to several people familiar with his plan, that economic warfare was the key to the overthrow of the government in Tehran. One person briefed on Mr. Nader’s activities said he tried to persuade Mr. Kushner to endorse the plan to Crown Prince Mohammed in person on a trip to Riyadh, although it was unclear whether the message was delivered.

Asked about Mr. Nader’s plans to attack Iran, the senior Saudi official said Mr. Nader had a habit of pitching proposals that went nowhere.

Mr. Nader was also in discussions with Mr. Prince, the former head of Blackwater, about a plan to get the Saudis to pay $2 billion to set up a private army to combat Iranian proxy forces in Yemen.

Since entering the White House, Mr. Trump has allied himself closely with Saudi Arabia and the Emirates. His first overseas trip was to Riyadh. He strongly backed Saudi and Emirati efforts to isolate their neighbor Qatar, another American ally, even over apparent disagreement from the State and Defense Departments.

This month, Mr. Trump also withdrew from an Obama administration nuclear deal with Iran that both Saudi Arabia and the United Arab Emirates had campaigned against for years, delivering them their biggest victory yet from his administration.