WASHINGTON — The embattled F.B.I. agent who oversaw the opening of the Russia investigation mounted an aggressive defense of himself and the F.B.I. on Thursday, rejecting accusations that he let his private political views bias his official actions and labeling Republicans’ preoccupation with him “another victory notch in Putin’s belt.”

“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,” the agent, Peter Strzok, told House lawmakers investigating what Republicans say is evidence of rampant bias at the top levels of the F.B.I.

But in defending himself and his agency, Mr. Strzok had to weather hours of blistering attacks by Republicans, who accused him not only of personal animus toward President Trump but also of blatant lying and moral misconduct with a senior F.B.I. lawyer, Lisa Page.

It was a remarkable day of shouting matches and personal attacks that showcased the deep partisan divide over the federal investigation into whether the Trump campaign colluded with Russia to influence the outcome of the 2016 election. The performance by Republicans, echoing Mr. Trump’s own lines of attack, demonstrated just how far many in the party have moved since the days when they were seen as the party of law enforcement, deferential to its power and prerogatives.