Sen. Tim Kaine Timothy (Tim) Michael KaineButtigieg stands in as Pence for Harris's debate practice Trump meets with potential Supreme Court pick Amy Coney Barrett at White House Names to watch as Trump picks Ginsburg replacement on Supreme Court MORE (D-Va.) said Saturday that the FBI has become "a leaky sieve," accusing FBI employees of "actively working to help" Republican presidential nominee Donald Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE's campaign.

Kaine seized on former New York City Mayor Rudy Giuliani's recent comment that he "heard about" the FBI's review of newly discovered emails linked to Democratic nominee Hillary Clinton Hillary Diane Rodham ClintonButtigieg stands in as Pence for Harris's debate practice Senate GOP sees early Supreme Court vote as political booster shot Poll: 51 percent of voters want to abolish the electoral college MORE's use of a private email server while secretary of State in advance.

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Giuliani, an adviser to Trump, clarified to CNN on Friday that no current FBI officials tipped him off about new developments into the bureau's probe of Clinton's server.

“I don’t think Giuliani’s walkback is credible,” Kaine, the Democratic vice presidential nominee, said during an interview with Fusion on Saturday.

"I think the FBI sadly has become like a leaky sieve," Kaine continued.

“What’s come out since,” Kaine said, “suggests that it’s probably more likely explained that [Comey] knew that the FBI is not only a leaky sieve but there were people within the FBI actively working — actively working — to try to help the Trump campaign."

"This just absolutely staggering, and it is a massive blow to the integrity of [the FBI]," he said.

FBI Director James Comey has received hefty criticism after sending a letter to Congress last week, alerting lawmakers that the agency discovered emails that may be "pertinent" to the investigation into Clinton's server.

“I don't think Director Comey is trying to influence the outcome of the election," said Kaine. "I think he made a massive mistake in judgment in violating two protocols two Fridays ago, talking about a new angle to the email story."

Following the letter, The Wall Street Journal reported intense dissension within the FBI over the investigation into Clinton's server.

Comey announced in July that he would not be recommending charges against the former secretary of State and that the FBI had completed its investigation.

"And so he probably felt pressure to do something because he felt like subordinates were going to do it if he didn't," said Kaine. "That doesn't justify what he did. I still think it was a mistake in judgment."