In Thursday's hearing of the House Judiciary and Oversight Committees, Republicans hope to further undermine the credibility of the investigation and head off any potential effort to impeach Trump.
In response, Sen. Bob Goodlatte (R-Virginia) snapped, "Mr. Strzok, you are under subpoena and are required to answer the question".
"Let me be clear, unequivocally and under oath: Not once in my 26 years of defending my nation did my personal opinions impact any official action I took", Strzok said.
He will also hit back at President Trump's characterizations of Mueller's work and the threat of Russian election interference, saying, "This investigation is not politically motivated".
To Democrats deriding the committees for their investigation into allegations of bias at the FBI, Goodlatte said "the American people hope you will understand that this investigation goes to the very heart of our system of justice, one that is supposed to be fair and treat everyone equally under the law".
Gowdy pressed him on why Mueller removed him from the Russian Federation probe if Strzok was not biased in any way.
"As House Judiciary Republicans spend half the day or longer publicly flogging Peter Strzok", wrote Wheeler, "know that all that flogging can not change the fact that key evidence in Mueller's possession, evidence which I suspect implicates the president directly, has absolutely no tie to Peter Strzok at all". "But to suggest somehow we can parse down the words of shorthand, text actual conversations like they're some contract for a vehicle is simply not consistent with my or most people's use of text messaging". "If we have a problem with this policy we should take it up with the Federal Bureau of Investigation, not badger Mr. Strzok".
Strzok said in the hearing, convened by two Republican-controlled House of Representatives committees: "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".
"But the thought of exposing that information never crossed my mind", Strzok said.
For a brief moment, lawmakers floated the idea of Strzok having some company when he appears Thursday - as the committee offered Page a spot to testify at Thursday's hearing. "We'll stop it", Mr. Strzok replied.
Strzok told lawmakers that he was overseen by bureaucratic layers of oversight that would have stopped him from meddling in the election, even if he had wanted to do so.
In an interview with investigators with the Justice Department's Inspector General, Page said she was "totally appalled that the President would insult the father of a dead service member".
The contentious hearing follows hours of closed-door questioning last week.
FBI Director Christopher Wray says the FBI has referred to internal disciplinary officials employees who were singled out for criticism in the inspector general's report.
"Like many people, I had and expressed personal political opinions during an extraordinary presidential election", Strzok said.
"Ex-FBI LAYER Lisa Page today defied a House of Representatives issued Subpoena to testify before Congress!"