Updated at 10:04 a.m. ET
Republican attacks on federal law enforcement have helped the Russian campaign to sow chaos within the United States, an embattled top FBI counterintelligence agent told Congress on Thursday.
"Russian interference in our elections constitutes a grave attack on our democracy," Deputy Assistant Director Peter Strzok told lawmakers in his prepared opening statement.
"Most disturbingly, it has been wildly successful — sowing discord in our nation and shaking faith in our institutions. I have the utmost respect for Congress's oversight role, but I truly believe that today's hearing is just another victory notch in [Russian President Vladimir] Putin's belt and another milestone in our enemies' campaign to tear America apart."
Strzok's statement also denied that political considerations have ever affected his official acts as an investigator.
The tone he set might not endear him with the leaders of the House Judiciary and oversight committees, which have made him and a former FBI lawyer, Lisa Page, the targets of months of attacks about political bias within the agencies.
Lawmakers might also be irked by Strzok's caveat that he does not expect to be able to answer all their questions because some will likely involve the ongoing Russia investigation and some may touch on matters that FBI lawyers have forbade him to discuss.
Strzok played important roles in both the investigation into Hillary Clinton's private email server while she was secretary of state and the investigation of Russian election interference in the 2016 presidential race now run by Justice Department special counsel Robert Mueller.
Mueller reassigned Strzok after the Justice Department discovered highly political text messages that he sent Page during the 2016 election cycle.
Page told investigators that she and Strzok were having an affair, according to a Justice Department Inspector General's report about the Clinton email investigation, and that they used their work devices to conceal that from their spouses.
That meant Justice Department investigators could access all their messages, including the ones in which they criticized Trump and appeared to discuss how they might use their powers to keep him from being elected.
The two were faulted by the official investigation into the conduct of the Clinton email inquiry. Page has since left the bureau. Strzok remains with the FBI, albeit removed from his old investigative job.
Republicans have cited Page's and Strzok's messages as evidence about what they call bias inside federal law enforcement and that conspirators launched the Russia investigation out of partisan animus.
Strzok has met with members of the House Judiciary Committee privately, but his hearing scheduled for 10 a.m. on Thursday is open.
Members of the House oversight committee also are scheduled to attend and question him.
Judiciary Committee Chairman Rep. Bob Goodlatte, R-Va., issued a subpoena to compel Page to talk to lawmakers Wednesday, but her attorney said she would not appear. Goodlatte blasted that decision on Wednesday, saying "none of her excuses hold water."
The leaders of the FBI and the Justice Department have defended the sum of the agencies' work and tried to distance themselves from the embarrassment caused by the discovery of the messages.
"We were deeply troubled by text messages exchanged between Strzok and Page that potentially indicated or created the appearance that investigative decisions were impacted by bias or improper considerations," the Justice Department's inspector general wrote in its June report.
The Strzok-Page saga has also attracted the personal attention of the president, who has frequently commented on the ongoing scandal as evidence of a "witch hunt" against him.
Trump criticized the two again ahead of the House hearing in a Twitter post from his trip to Europe.
"Ex-FBI LAYER Lisa Page today defied a House of Representatives issued Subpoena to testify before Congress! Wow, but is anybody really surprised! Together with her lover, FBI Agent Peter Strzok, she worked on the Rigged Witch Hunt, perhaps the most tainted and corrupt case EVER!"