B. Todd Jones (C), President Obama's intended nominee for director of the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives, listens to Obama's long-awaited gun control proposal in the Eisenhower Executive Office Building Jan. 16, 2013. (Michael Reynolds/EPA)

An influential Republican senator has threatened to put on hold President Obama’s nominee to head the Bureau of Alcohol, Tobacco, Firearms and Explosives, raising once again the possibility that the gun-regulating agency will not have a director.

The president two weeks ago nominated acting ATF Director B. Todd Jones to become the agency’s full-time director. The ATF has been without a director for six years. But on Thursday, Sen. Charles E. Grassley (Iowa), the ranking Republican on the Senate Judiciary Committee, released a statement criticizing Jones and questioning his ability to lead the ATF. Grassley criticized Jones’s decisions in a Minnesota legal case involving housing discrimination and raised questions about an ATF operation in Milwaukee. He also criticized Jones for not answering questions from his staff about his decisions at ATF in the aftermath of the bungled Phoenix gun operation known as Fast and Furious.

“I believe that ATF needs a Senate-confirmed director,” Grassley said at a Senate Judiciary Committee business meeting. “However, if we are prohibited from asking questions about important matters that get to the core of leadership, character and candor about a nominee’s ability to run an agency, it makes our job that much harder.”

Jones became the acting director of the ATF after his predecessor resigned amid the pressure connected to Fast and Furious. He has served as a part-time director, while also serving as the U.S. attorney in Minnesota.

Several former and current ATF officials have praised Jones’s leadership of the agency. But a former FBI special agent in charge in Minneapolis has sent members of the Senate Judiciary Committee a letter sharply criticizing Jones’s leadership and accusing him of declining to prosecute cases involving gangs, drugs and gun crimes.

In his letter, which was first reported by a local Fox television station in Minneapolis, Donald Oswald said that Jones has an “atrocious professional reputation” and is motivated by “personal political gain.”

The ATF has been without a director since the position was first required to be confirmed by the Senate in 2006. President George W. Bush’s first nominee, Michael J. Sullivan, was blocked by three Republican senators who accused Sullivan of not doing enough to end the ATF’s “overly burdensome regulatory policies” on gun owners.

The National Rifle Association vowed to fight Obama’s first nominee, Andrew Traver, who headed the ATF’s Chicago field office, accusing him of being “deeply aligned with gun-control advocates and anti-gun activities.”