Senate Judiciary Committee Chairman Dick Durbin doubled down Monday on his support for President Biden's nominee to run the bureau of Alcohol Tobacco and Firearms (ATF) after Republicans asked the committee to hold an extra hearing on David Chipman's nomination to examine bias allegations against him. 

Durbin, D-Ill., said the allegations against Chipman, in a letter led by committee Ranking Member Chuck Grassley, R-Iowa, are part of "vitriolic, baseless attacks against his highly credentialed nominee." The committee won't call Chipman back for another hearing, Durbin said. 

"ATF needs a Senate-confirmed leader, and Mr. Chipman is well-qualified and has been extensively vetted for the role. He deserves to be confirmed," Durbin said in a response letter. "And it is my fervent hope that Committee Republicans will quit embracing anonymously-sourced efforts to smear Mr. Chipman."

The comments come as Chipman's nomination is stalled in the Senate. He received a party-line vote in the Judiciary Committee, which empowers Democrats, who are in the majority, to discharge his nomination and bring it to the floor for a final vote. 

David H. Chipman appears before a Senate Committee on the Judiciary hearing for his nomination to be director of the bureau of Alcohol, Tobacco, Firearms and Explosives in the Dirksen Senate Office Building in Washington, D.C., Wednesday, May 26, 2021. (Rod Lamkey / CNP/Sipa USA) (Rod Lamkey / CNP/Sipa USA)

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But several Senate Democratic Caucus members still haven't publicly announced whether they'll support Chipman's confirmation. They include Sens. Angus King, I-Maine, Kyrsten Sinema, D-Ariz., Joe Manchin, D-W.Va., John Hickenlooper, D-Colo., and Jon Tester, D-Mont. 

King is still considering the nomination, spokesman Matthew Felling told Fox News on Tuesday. So is Tester, his spokesman Andy Bixler said Tuesday. 

"As a proud gun owner, Senator Tester believes ATF needs a strong leader to support the agency’s law enforcement mission," Bixler said. "Senator Tester will continue to review David Chipman’s record and testimony to ensure he would support our brave law enforcement officers and respect Montanans’ Second Amendment rights."

Hickenlooper was undecided last month as well, the Wall Street Journal reported. And Manchin and Sinema also haven't publicly taken any stance on Chipman as they've been highly involved in negotiating a bipartisan infrastructure bill in recent weeks. 

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With a 50-50 Senate, Chipman's margin for error in the Senate is nearly nonexistent. He will almost certainly need all 50 Democrats – and therefore all five of those who are currently undecided – to be confirmed, barring a surprise "yea" vote from a Republican. And at least one moderate Republican, Sen. Susan Collins, R-Maine, who has voted for many Biden nominees, already announced her opposition to Chipman. She called him "unusually divisive." 

The GOP-backed letter Durbin was referring to cited two reports – one from the Daily Caller and another from The Reload – that indicated Chipman may have made racist comments while he was an ATF agent. Fox News has not independently confirmed these allegations. Chipman left the ATF about nine years ago and has since worked with gun control groups. 

The Republicans' letter cited two Equal Employment Opportunity Commission (EEOC) complaints made against Chipman while he was at the ATF. Fox News previously reported on the existence of the complaints. During his confirmation hearing, Chipman said those "complaints were resolved without any finding of discrimination and no disciplinary action was taken against me." 

But the EEO complaints are far from the only controversy Chipman has faced during his confirmation process. Last year on Cheddar News Chipman mocked first-time gun owners.

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"They might think that they're die-hard ready to go," Chipman said of first-time gun owners. "But unfortunately they're more like Tiger King and they're putting themselves and their families in danger."

Chipman then said people who bought guns during a surge in sales in early 2020 should "secure that gun locked and unloaded and hide it behind the cans of tuna and beef jerky" and "only bring that out if the zombies start to appear." 

The nominee also said in a 2019 Reddit "Ask Me Anything" that because people who fail background checks while trying to buy guns often later are caught unlawfully using a gun, "This is a perfect opportunity to arrest people before committing crimes."

Sen. Mike Lee, R-Utah, called Chipman's remarks "the sort of thing that's reserved for bad post-apocalyptic dystopian novels and movies."

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There are also allegations against Chipman that he lost his service weapon, although those are still unverified and the ATF has told Fox News it has no record of Chipman ever misplacing his firearm. 

It is unclear when Chipman's nomination could come up for a vote in the Senate. It is unlikely that Majority Leader Chuck Schumer, D-N.Y., will call a vote before he knows Chipman has the votes to be confirmed.

The White House has remained behind Chipman. Spokesperson Michael Gwin previously told Fox News that Chipman "served honorably in law enforcement for 25 years – promoted into positions of trust and management at the ATF, participating in complex investigations, and putting himself in harm’s way as a member of the Bureau’s SWAT team."

Gwin also recently told the Wall Street Journal that the president "looks forward to the Senate quickly voting on Chipman’s nomination as ATF Director."

Fox News' Marisa Schultz and Caitlin McFall contributed to this report.