House Republican leaders made the hearing the day’s top priority, postponing floor votes so that the Committee on Oversight and Government Reform could continue without interruption. The Obama administration appeared focused on the testimony, with senior officials at the White House, the State Department and the Pentagon responding through the day to Republican accusations of incompetence and cover-up in campaign war room style.

In the balance, in the view of both Democrats and Republicans, is not just the reputation of Mr. Obama but also potentially the prospects for the 2016 presidential election as well, since Mrs. Clinton, who stepped down in February, is the Democratic Party’s leading prospect. If the testimony did not fundamentally challenge the facts and timeline of the Benghazi attack and the administration’s response to it, it vividly illustrated the anxiety of top State Department officials about how the events would be publicly portrayed.

Mr. Hicks offered an unbecoming view of political supervision and intimidation inside the Obama administration. When Representative Jason Chaffetz, Republican of Utah, visited Libya after the attack, Mr. Hicks said his bosses told him not to talk to the congressman. When he did anyway, and a State Department lawyer was excluded from one meeting because he lacked the necessary security clearance, Mr. Hicks said he received an angry phone call from Mrs. Clinton’s chief of staff, Cheryl Mills.

“So this goes right to the person next to Secretary of State Clinton. Is that accurate?” asked Representative Jim Jordan, Republican of Ohio. Mr. Hicks responded, “Yes, sir.”

A State Department official said Mr. Hicks had been free to talk to Mr. Chaffetz, but that department policy required a department lawyer to be present during interviews for any Congressional investigation.