Peter Navarro, a trade adviser to former President Donald J. Trump who helped lay plans to keep Mr. Trump in office after the 2020 election, was sentenced on Thursday to four months in prison for defying a subpoena from the House committee investigating the Jan. 6, 2021, attack on the Capitol.

Mr. Navarro, 74, was found guilty in September of two misdemeanor counts of criminal contempt of Congress. The judge overseeing the case, Amit P. Mehta, rejected Mr. Navarro’s primary defense: that Mr. Trump had personally directed him not to cooperate with the subpoena, and that he believed he was shielded by executive privilege.

“The words executive privilege are not magical incantations,” Judge Mehta said.

In a testy exchange with Mr. Navarro’s lawyers beforehand, the judge singled out Mr. Navarro’s decision to flout the subpoena even as other aides to Mr. Trump negotiated whether to comply. “I have a great deal of respect for your client and what he’s achieved professionally, I do,” he added. “Which makes it all the more disappointing, the way he behaved.”

Mr. Navarro, a Harvard-trained economist and a vocal critic of China, served as a trade adviser to Mr. Trump before turning his focus to the pandemic response. After the 2020 election, however, he increasingly explored ways to subvert the outcome of the race and keep Mr. Trump in power.

Mr. Navarro, along with Stephen K. Bannon, a longtime adviser to Mr. Trump, devised a plan known as the Green Bay Sweep. Under the strategy, they would try to delay certification of the election by persuading Republican lawmakers to repeatedly challenge the results in various swing states and apply pressure on former Vice President Mike Pence to discredit the outcome. He also cast doubt on the results of the race, compiling instances of purported irregularities and issuing a three-part report claiming election fraud as part of what he described as an “immaculate deception.”

Those efforts ultimately elicited the attention of the House committee, which sought documents and testimony from Mr. Navarro. He repeatedly spurned those requests.