However, according to his testimony, Mr. Melton said on Twitter and other forums that Nicholas was responsible for “100 percent” of Badger’s technology, describing GM as little more than a supportive player. GM withdrew from the partnership shortly after the Hindenburg report, and the Badgers were never produced.

Other Nikola executives, including Mark Russell, chief executive, and Kim Brady, chief financial officer, have repeatedly warned Melton that his statements could backfire. The executives even made an “intervention” in which they tried, without success, to convince Mr. Milton of the truth, according to witnesses.

Mukasey, the defense attorney, referred to internal emails to Nicolas in which executives praised Milton’s media appearances. “They were telling Trevor what a good job he was doing,” Mukasey said.

The convictions include accusations that Mr. Melton defrauded a man who sold him a farm in Utah. Mr. Melton paid for the farm in part with Nicholas stock options that proved worthless.

An acquittal of one of the securities fraud charges provided some relief for Mr. Milton. A conviction on this charge would have subjected him to a maximum sentence of 25 years.

Nikola, based in Phoenix, continues to operate, producing a limited number of battery-powered trucks in collaboration with established companies including IVECO, the Italian truck maker that makes Nikola cars in Germany. Michael Loeschler, an auto industry veteran who was formerly CEO of German carmaker Opel, was named Nikola’s president in August. .

Stefan Hartung, CEO of German electronics maker Bosch, said in an interview this month that he still believes in Nikola. Bosch, one of the world’s largest manufacturers of components for the automotive industry, supplies fuel cell technology to the Nikola trucks being developed.