Questioning amounted to a live fact-checking of the White House's claims this week, with McCabe undercutting some of Trump's most stringent arguments.
Adding to the drama, Deputy Attorney General Rod Rosenstein, who the White House had been portraying as the person who called for Comey's firing, walked into the Senate Hart Office Building
just feet from the McCabe hearing for a meeting.
Here are five things we learned from another wild day built off the investigations into Russia's possible coordination with the Trump campaign.
Among the many strange things Trump did this week, alleging that Comey told him three separate times
that he was not the subject of investigation is near the top of the list.
The very first question of the hearing, from committee Chairman Richard Burr, was whether Comey actually made that promise to Trump. McCabe said he could not directly answer that question.
Later, Democratic New Mexico Sen. Martin Heinrich asked: "Is it your experience that people who are innocent of wrong doing typically need to be reassured that they're not the subject of an investigation?"
"No sir," McCabe replied.
Sen. Susan Collins, a moderate Republican from Maine, peppered McCabe with questions of whether the FBI would ever do something like alert someone who is not the subject of an FBI probe that, in fact, they are not the subject of an FBI probe.
McCabe said "It is not" something he would ever do.
2. McCabe had more bad news for the White House
The top Democrat on the Senate Russia investigation, Virginia's Mark Warner, asked McCabe if he would alert Senate investigators if the White House tries to interfere in the Russia investigation or any other probe.
McCabe said he would.
Not long after that, McCabe undercut the White House on its argument that Comey was doing a bad job at the FBI and had lost the trust and support of rank-and-file FBI agents.
Heinrich checked that claim.
"No, that is not accurate," McCabe told Heinrich.
3. Rosenstein made an entrance at the most auspicious time possible
Rosenstein -- who the White House had pinned Comey's firing on
until Trump himself took full credit for it Thursday
-- walked into a private meeting with Burr and Warner just feet from the McCabe hearing.
For 45 minutes, all attention turned to the secure room in the Senate Hart Office Building, where one of the three men behind Comey's firing sat.
Burr said afterward that Comey's firing did not specifically come up. But Warner pressed Rosenstein about appointing a special prosecutor to oversee the Russia investigation.
Asked later if he was comfortable with Rosenstein overseeing the Russia investigation -- California's Sen. Dianne Feinstein, a veteran Democrat on the intelligence committee, called for Rosenstein's recusal earlier -- Burr offered a backhanded deflection.
"We don't have the luxury of choosing who we work with," Burr said.
4. The White House has not made any effort to impede the Russia investigation, McCabe says
McCabe undercut a series of top White House talking points in the wake of the Comey firing. But he also had a reality check for enraged Democrats, beating back some of the most explosive reports about the firing.
"The work of the men and women of the FBI continues despite any changes in circumstance, any decisions," McCabe told Sen. Marco Rubio. "So there has been no effort to impede our investigation today."
McCabe was also clear that they did not need additional resources for their Russia investigation -- and threw cold water on reporting that Comey asked Justice Department for additional help for the Russia investigation
before being fired.
5. Trump still appears to be the only official who does not believe Russia messed with the US elections
Trump has consistently questioned the extensive evidence that Russians actively meddled in the 2016 election to help put him in the White House, calling it "fake" and a "hoax" at various points
Thursday's hearing offered yet more evidence that Trump stands alone on this.
"For the whole panel as the assembled leadership of the intelligence community, do you believe that the January 2017 intelligence community assessment accurately characterized the extent of Russian activities in the 2016 election and its conclusion that Russian intelligence agencies were responsible for the hacking and leaking of information and using misinformation to influence our elections?" Warner asked the panel.
And the leader of Trump's intelligence operations all said: "Yes."