After former FBI Director James Comey testified again Monday before a House panel about the 2016 FBI investigation into Hillary Clinton's private email server, he talked with reporters outside the room, weighing in on President Trump's recent tweet calling Michael Cohen "a rat."

"It undermines the rule of law," Comey said. "This is the president of the U.S. calling a witness who is cooperating with his own Justice Department a 'rat.' Say that again to yourself at home and remind yourself where we ended up. This is not about Republicans and Democrats. This is about — what does it mean to be an American? ...There's a set of values that represent the glue of this country, and they are under attack by things just like that. We have to stop being numb to it."

Comey also had harsh criticism for Republican lawmakers, who he urged to "stand up and speak the truth — not be cowed by mean tweets or fear of their base. There is a truth and they're not telling it." He added, "Their silence is shameful."

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He told reporters that he'd "rather not be talking" to journalists, but "somebody has to stand up for the rule of law. I hope there's a whole lot more somebodies out there than just me."

White House press secretary Sarah Sanders responded in a tweet that "Republicans should stand up to Comey and his tremendous corruption - from the fake Hillary Clinton investigation, to lying and leaking, to FISA abuse, and a list too long to name." She also called him a "shameless fraud" and said the president had done the country "a service by firing him and exposing him for the shameless fraud he is."

The former FBI director was also asked by reporters whether he has confidence in Acting Attorney General Matt Whitaker. "No comment," he replied.

Comey was testifying before the House Judiciary and Oversight Committees in a closed session. "The questions about Hillary Clinton and the Steele dossier strike me as more of the same," he said. "I didn't learn anything new in there. Maybe they did." It was Comey's second appearance before lawmakers this month as part of an effort by Republicans who are pressing for answers to questions about Hillary Clinton and her private email server in 2016. As was the case in his Dec. 7 appearance, a transcript is expected to be released after this session.

An attorney with the Justice Department told members of the House Judiciary Committee during the session earlier this month that the fired FBI director would not answer some of their questions, which led to frustration among Republicans. Democrats called that hearing a waste of time.

According to the transcript of his earlier appearance, Comey did not respond to some questions related to special counsel Robert Mueller's investigation into Russian interference in the 2016 election. Comey was advised by Cecelia Bessee, an attorney representing the FBI.

Rep. Jerry Nadler, D-New York,who is likely to chair the Judiciary Committee in the next Congress, has said he would end this investigation — which is officially supposed to be examining DOJ and FBI actions in the lead-up to the 2016 elections — when he takes over the committee chairmanship in January. He called it "a waste of time to start with" and said that the "entire purpose of this investigation is to cast aspersions on the real investigation which is Mueller."

Since then, Comey, in a public appearance in New York, urged Americans to "get off the couch" and vote to deny President Trump re-election in 2020, referring to Mr. Trump's presidency as an "attack on our values." He also said in that appearance that he would be "fine" if Congress were to vote to impeach and remove Mr. Trump from office.

Grace Segers contributed to this report.