Peter Strzok’s anti-Trump text messages have created credibility problems for the former FBI official that could potentially spoil evidence in the special counsel’s Russia investigation, Deputy Attorney General Rod Rosenstein acknowledged in a congressional hearing Thursday.

“You need to consider any issues that go to the credibility of the witness or the credibility of the evidence,” Rosenstein said during a line of questioning with Texas Rep. John Ratcliffe, a member of the House Judiciary Committee.

He then added that if special counsel Robert Mueller “were to rely upon a document that Mr. Strzok produced, or a statement that he took, or to call him as a witness,” he “obviously he would need to consider…evidence that would tend to impeach his credibility.” (RELATED: Strzok: ‘We’ll Stop’ Trump Presidency)

Ratcliffe was pressing Rosenstein over Strzok’s texts and his role on the collusion investigation, which was formally opened on July 31, 2o16. As deputy chief of counterintelligence, Strzok was the top investigator on the FBI’s investigation into possible collusion between the Trump campaign and Russian government.

He joined Mueller’s team shortly after it was formed on May 17, 2017. Strzok was kicked off of the investigation after the discovery of anti-Trump text messages that he exchanged with his mistress, an FBI lawyer named Lisa Page. (RELATED: Democrats Keep Interrupting Jim Jordan As He Grills Rod Rosenstein)

According to Ratcliffe, Strzok said in a closed-door deposition on Wednesday that he drafted the initial investigative plan on the Russia collusion investigation and handled witnesses and evidence. Strzok was also one of the two FBI agents who interviewed then-national security advisor Michael Flynn on Jan. 24, 2017 at the White House.

Flynn has since pleaded guilty to lying during that interview about his contacts with Russian ambassador Sergey Kislyak in December 2016. Some Republican lawmakers have questioned the guilty plea because of initial reports that some in the FBI did not believe that Flynn had lied during the interview.

“If your root evidence is fairly called into question, everything that comes from that evidence is fairly called into question,” said Ratcliffe, a former federal prosecutor.

“Yes,” Rosenstein agreed.

Rosenstein said that he was troubled by Strzok’s text messages, which were initially discovered by the Justice Department’s Office of the Inspector General.

In one message sent on Aug. 8, 2016, Strzok told Page that “we’ll stop” Trump from becoming president. In another, Strzok called Trump and idiot.

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