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Some of the key moderate Democratic senators needed to confirm President Biden's pick to run the Bureau of Alcohol, Tobacco and Firearms, David Chipman, are still undecided about whether they will vote for the controversial nominee who has a history of working with high-profile gun control groups. 

Chipman, who also has a history of undiplomatic comments about guns, gun owners and how to enforce gun laws, is staunchly opposed by most Republicans. 

Even Sen. Susan Collins, R-Maine, one of the Senate Republicans who has been willing to vote for many of the president's nominees, said she will vote against the "unusually divisive" Chipman. She said this is because he is "an outspoken critic of the firearms industry and has made statements that demean law-abiding gun owners."

That means there is a significant chance Chipman's nomination could come down to a handful of Democrats, like Sen. John Tester of Montana. 

In this Sept. 25, 2019, file photo, former Giffords Law Center Senior Policy Advisor David Chipman speaks at a House Judiciary Committee hearing on assault weapons on Capitol Hill in Washington. Chipman is nominated to run the Bureau of Alcohol, Tobacco and Firearms. (AP Photo/Andrew Harnik, File) (AP)

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"As a proud gun owner, Sen. Tester believes ATF needs a strong leader to support the agency’s law enforcement mission," Tester communications director Sarah Feldman told Fox News. "Sen. 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."

Several other moderate Democrats from red or purple states haven't taken public stances on the nomination and did not reply to inquiries from Fox News about where they stand on Chipman. This group includes Sens. Joe Manchin, D-W.Va., Kyrsten Sinema, D-Ariz., Maggie Hassan, D-N.H., and Angus King, I-Maine.

Manchin was asked about Chipman's nomination last month and acknowledged that "there's a lot of controversy," according to Politico. "I really am undecided," he said.

Sen. Lisa Murkowski, R-Alaska, who represents the other major Republican swing vote after Collins, has not announced a position on Chipman's nomination. Her support, or a vote from any other Republican, would be a big boost to Chipman's chances. But even if Murkowski votes against him, at least one Democrat would need to as well in order to block Chipman's confirmation. Otherwise, Vice President Harris will be able to break a tie in the 50-50 Senate. 

Sen. Jon Tester, D-Mont., in the House Chamber of the U.S. Capitol in Washington, Feb. 4, 2020. Tester's office told Fox News he is still evaluating Biden ATF nominee David Chipman's record.  (Reuters)

Several elements of Chipman's history are leading Republicans to fervently oppose him and moderates to carefully consider their decision. Among them are his past support for very strict regulation of AR-15s, his work for the major gun-control organizations Giffords and Everytown, and his past charged comments about gun owners and gun enforcement. 

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"I had to train repeatedly to be proficient in the use of a gun and most of the new buyers who went to their gun store and bought a gun have no training whatsoever," Chipman said on Cheddar News last year about a spike in gun purchases early in the pandemic. 

"In their mind they might be confident. They might think that they're die-hard ready to go. But unfortunately they're more like Tiger King and they're putting themselves and their families in danger," Chipman added. "What I would suggest is for those people who were first-time gun owners… I would secure that gun locked and unloaded, and hide it behind the cans of tuna and beef jerky that you have stored in the cabinet and only bring that out if the zombies start to appear." 

And in a 2019 Reddit "Ask Me Anything," Chipman appeared to advocate for arresting people before they commit crimes – something he walked back when pressed about it in his Senate Judiciary Committee hearing. 

"While at ATF I conducted studies involving people who failed background checks to determine how many later committed crimes with a gun – many did," Chipman said on the Reddit forum. "This is a perfect opportunity to arrest people before committing crimes rather than responding after the fact."

"We don't arrest people before they commit crimes. That's the sort of thing that's reserved for bad post-apocalyptic dystopian novels and movies," Sen. Mike Lee, R-Utah, said of that comment during Chipman's Judiciary Committee markup.

Sen. Mike Lee, R-Utah, has led Republican resistance to Biden ATF nominee David Chipman.  (AP)

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The White House has defended Chipman, who advanced out of the Judiciary Committee late last month on a party-line vote.

"David 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," White House spokesman Michael Gwin told Fox News about Chipman last month. 

Biden so far in his presidency has enjoyed significant loyalty from Democratic senators on his nominees, managing to confirm multiple individuals who Republicans unanimously opposed. The only Biden nominee who saw a hearing and then failed to get confirmed was Neera Tanden, the former nominee to run the Office of Management and Budget. 

Manchin announced his opposition to Tanden after her confirmation hearing and before her committee vote, largely citing her caustic Twitter history that included broadsides against a number of senators. 

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Chipman's nomination has also tipped off massive pressure campaigns both for and against his confirmation, with outside groups targeting moderate senators who could provide the pivotal swing vote either way. 

The National Rifle Association announced last week that it is spending $250,000 on television ads in West Virginia to pressure Manchin to vote against Chipman. Giffords, meanwhile, has spent at least $150,000 on pro-Chipman advertisements while Everytown is directing activists to contact their senators in support of Chipman.

It's unclear exactly when Senate Majority Leader Chuck Schumer, D-N.Y., will bring Chipman's nomination to the floor, but it is likely to happen this month. 

Fox News' Caitlin McFall and Morgan Phillips contributed to this report.