Chuck Canterbury, President Trump's nominee to serve as head of the Bureau of Alcohol, Tobacco, and Firearms, is likely to see his nomination withdrawn due to stringent opposition from Republican members of the Senate Judiciary Committee, the Washington Free Beacon learned on Thursday.

"Chuck Canterbury does NOT have sufficient support in the Judiciary Committee and is now expected to be held up instead of getting a vote today," a GOP Senate aide told the Free Beacon on Thursday morning. "Republican members are (1) Concerned about his stances on gun control, and (2) there is still some bad blood for him flipping the Fraternal Order of Police in support of the First Step Act (after opposing the more moderate version), allegedly in exchange for the ATF Director nomination."

"His 2A views are bad and he'd lose a lot of votes in committee," a second Senate source told the Free Beacon. "We expect the White House will pull him given lack of support from Republicans on the Judiciary Committee."

At least one Republican senator, Utah's Mike Lee, seemed to agree with this assessment when reached for comment.

"Sen. Lee has concerns about Canterbury's Second Amendment views and is pleased the markup has been delayed," Lee's office told the Free Beacon.

Canterbury, who served as the president of the national Fraternal Order of Police (FOP), was expected to receive a vote out of committee Thursday morning. But his nomination was blocked by conservative Republicans who voiced concerns about his stances on gun control.

Canterbury's nomination seemed in trouble during his July confirmation hearing when he frustrated Republican senators by refusing to deviate from the official positions of the FOP on gun control. He avoided answering questions about his views on the Second Amendment and even explicitly said he did not understand ATF regulations well enough to say what gun control policies he could or could not implement.

"If you're not familiar with the process running the ATF, then you are not qualified," Sen. John Kennedy (R., La.) told him at the time.

It was Canterbury's views on gun control, however, that sealed the deal. His past support for universal background check proposals and for President Obama’s attorney general Eric Holder were both major stumbling blocks, according to the second Senate source.

While head of the FOP, Canterbury explicitly supported background checks. In a 2013 letter, he told then-Senate Judiciary chairman Patrick Leahy (D., Vt.) that "expansion of background checks on firearm purchasers" was an "absolutely critical" element of "addressing gun violence."

"We believe the most logical starting point to address gun violence is the expansion of the background check system," Canterbury wrote on behalf of the FOP. "Incomplete or absent background checks create a gaping hole in the wall between firearms and criminals."

According to the Bureau of Justice Statistics, just 10.5 percent of gun violence offenders purchased their firearm through a transaction that would be covered by any expanded background check proposal. Additionally, a Free Beacon analysis earlier this year found Colorado's universal background check law had little effect on the number of checks done in the state, suggesting it was not as effective as advocates claim.

In 2009, Canterbury testified on behalf of then-nominee Holder, calling "his positions, his policy work, and the official acts … consistent with the goals of the FOP." Holder would go on to oversee operation Fast and Furious, a gun-running investigation where officials allowed guns to be sold to people connected with Mexican cartels in an effort to track their networks. The program was poorly overseen and led to the death of a Border Patrol agent. Critics argued the program's failure was due to Holder's negligence.

Opposition to Canterbury also stemmed from his convincing the FOP to change its position on the FIRST STEP Act, the White House's landmark criminal justice reform bill. The FOP initially opposed FIRST STEP, but changed its view after several revisions.

GOP Senate aides indicated that Canterbury was integral in bringing about this shift and that his work on FIRST STEP helped him clinch the ATF nomination, a view further indicated by Politico reporting in May. The Fraternal Order of Police did not immediately respond to a request for comment.

While GOP aides expect Canterbury's nomination to be withdrawn, that has yet to happen officially.

The White House did not immediately respond to a request for comment as to its intentions.

UPDATE 1:03 P.M., Thursday 26 September: This post has been updated to include comment from Senator Mike Lee (R., Utah).