Jeffrey Rosen: The acting attorney general was pivotal in holding off Trump’s effort to install a loyalist atop the Justice Department.

Jeffrey A. Rosen led the Justice Department during the final month of the Trump administration, when President Donald J. Trump and his allies most aggressively pressed federal and state officials to undo his loss in the 2020 presidential election.

Mr. Rosen, who served as acting attorney general, is expected to help the Jan. 6 committee paint a more detailed picture of efforts by Mr. Trump and his allies to weaponize the Justice Department in their efforts to overturn the election.

He is also expected to speak to the dramatic showdown that took place in the Oval Office on Jan. 3, 2021, when top Justice Department officials said they and others in the department would quit en masse if Mr. Trump replaced Mr. Rosen with Jeffrey Clark, who ran the department’s civil division and who was willing to help Mr. Trump in his efforts to undo the election.

In a statement he prepared for the committee, Mr. Rosen said that the Justice Department never found credible evidence of election fraud, underscoring one of the committee’s key points. “Some argued to the former president and public that the election was corrupt and stolen,” Mr. Rosen wrote. “That view was wrong then, and it is wrong today, and I hope our presence here today helps reaffirm that fact.”

While Mr. Trump’s efforts to install Mr. Clark atop the Justice Department have been well documented by previous investigations, including one by the Senate Judiciary Committee, Mr. Rosen could help the Jan. 6 committee to flesh out more information, including about Mr. Trump’s push for a special counsel to investigate voter fraud and his direct involvement in the pressure campaign on the Justice Department.

Mr. Rosen served as an official at the Transportation Department and the Office of Management and Budget during the George W. Bush administration, when he worked to rein in the federal government’s regulatory authority, which many Republicans saw as overreach. He returned to the Transportation Department under Mr. Trump and then joined the Justice Department to replace Rod J. Rosenstein, the deputy attorney general who oversaw the Russia investigation.

Mr. Rosen was overshadowed by Attorney General William P. Barr, who was close to Mr. Trump. But Mr. Barr left after the election, when the president attacked him for refusing to support his fraud claims, leaving Mr. Rosen to defend the department.