WASHINGTON -- In ways both overt and subtle, the acting director of the FBI undermined White House explanations for the firing of former Director James Comey.

Andrew McCabe, testifying before Congress Tuesday in place of his fired boss, contradicted the administration's characterizations of an investigation into potential co-ordination between Russia and the Trump campaign to influence the presidential election.

McCabe told lawmakers a counterintelligence investigation that a White House spokeswoman dismissed a day earlier as "one of the smallest things" on the FBI's plate was actually "highly significant." And though the White House has asserted Comey lost the backing of rank-and-file agents, McCabe flatly said, "that is not accurate."

"I can tell you that the majority, the vast majority of FBI employees, enjoyed a deep and positive connection to Director Comey," said McCabe, who called it the "greatest privilege and honour in my professional life to work with him." Comey, he added, "enjoyed broad support within the FBI and still does to this day."

McCabe's testimony undercut a White House narrative that has evolved in the two days since Comey was ousted. The White House initially said President Donald Trump made the call at the recommendation of the top two officials at the Justice Department, but in an NBC News interview that aired Thursday, the president said he would have fired Comey even without the recommendation.

Testifying before the Senate Intelligence Committee, McCabe repeatedly -- and at times bluntly -- rejected some of the claims the White House has used to explain the firing and to describe the investigation.

In one of the more dramatic exchanges, he was asked whether the investigation was -- as White House spokeswoman Sarah Huckabee Sanders maintained Wednesday -- a small investigation in relation to the other work the FBI is conducting.

"Sir," he told Sen. Angus King of Maine, "we consider it to be a highly significant investigation."

He also said he would refrain from discussing the investigation with the White House, pledged to report to the committee any efforts to interfere in it and said the dismissal of the director would do nothing to impede the probe.

"You cannot stop the men and women of the FBI from doing the right thing," he declared.

Trump maintains Comey told him that he was not under investigation, but McCabe suggested under questioning that was highly improbable.

He refused to comment on what Comey did or didn't say, but when asked whether it would be common for an FBI director to notify someone they were not the target of an investigation, said "I'm not aware of that being a standard practice."

McCabe's willingness to cut against White House talking points may reflect an understanding that he may not serve for long as acting director. The Justice Department seemed to be laying the groundwork for that possibility even before his congressional appearance, with leaders there interviewing four other candidates for the interim job Wednesday.

McCabe, a career FBI agent who ran the Washington field office and oversaw national security investigations out of headquarters, was a target of Trump's on the campaign trail following reports that his wife had accepted campaign donations from a close ally of Hillary Clinton during a failed bid for the state Senate.