WASHINGTON - A group of powerful Republicans in Congress is openly challenging B. Todd Jones' leadership of the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) as the Senate takes up President Obama's nomination to make the Minnesota lawyer the agency's permanent director.

A congressional letter that was signed by Sen. Charles Grassley, R-Iowa, and three leading House Republicans spells deepening trouble for Jones' nomination at a time when Republicans are aggressively challenging a number of Obama's second-term appointments.

As the ranking Republican on the Senate Judiciary Committee, Grassley is in a position to slow or even block Jones' nomination indefinitely, despite Democratic control of the Senate.

The letter to Jones, who also is the U.S. attorney for Minnesota, cites failures in a recent sting operation in Milwaukee that raise doubts about Jones' supervision as acting director. It adds to a chorus of GOP criticism over the administration's handling of the controversial ATF "Fast and Furious" gun tracking operation on the Mexican border, which Jones was brought in to clean up.

Earlier this week, Donald Oswald, a former Minneapolis FBI director and self-described Democrat, wrote to alert the Senate Judiciary Committee to Jones' "atrocious professional reputation within the federal law enforcement community."

Grassley has already threatened to hold up Jones' nomination in a separate dispute involving the Justice Department and the city of St. Paul's decision to drop a housing discrimination case that had been headed for the U.S. Supreme Court.

Under Senate rules, a single senator can block a nomination -- a move that can be overturned by a vote of 60 senators. That would require all Democrats plus the help of five Republicans. The White House referred inquiries about Grassley's letter to the ATF. Ginger Colbrun, an ATF spokeswoman, said only that the agency has received the letter and "it's being reviewed."

'Serious doubt'

Obama nominated Jones as part of 23 executive actions he issued last month to address gun violence, following the Sandy Hook elementary school massacre in Newtown, Conn.

Long viewed with suspicion by gun-rights advocates, the ATF has not had a permanent director since 2006, when Congress began requiring confirmation for the position.

The newest flashpoint involves operation "Fearless Distributing," a Milwaukee sting operation in which the ATF set up a phony storefront targeting gun traffickers. A recent Milwaukee Journal Sentinel investigation found what it called a string of mistakes, including a military-style machine gun that ended up on the streets, and a robbery of the agency store that netted $35,000 in merchandise.

Referencing the Sentinel story, the lawmakers' letter to Jones said: "We remain extremely concerned about your ability to manage ATF. If these reports are accurate, your ability to provide this leadership is in serious doubt."

Other signers were Reps. Darrell Issa, R-Calif., chairman of the House Committee on Oversight and Government Reform; Robert Goodlatte, R-Va., chairman of the House Judiciary Committee, and James Sensenbrenner, R-Wis., who chairs a House panel on crime, terrorism and homeland security.

Their concerns echo many of the criticisms leveled at Fast and Furious, in which the ATF allegedly lost track of scores of guns, including one found at the scene of a shootout that left a U.S. Border Patrol agent dead.

Grassley and Issa led GOP efforts to investigate Operation Fast and Furious, which resulted in a contempt vote in the U.S. House last summer against U.S. Attorney General Eric Holder.

Some analysts say the Jones' nomination now may become entangled in the larger battle between the Obama administration and congressional Republicans.

"I think that any nominee would be concerned when the Senate Judiciary Committee's ranking member and three powerful GOP House members express concerns like those," said University of Richmond law Prof. Carl Tobias, an expert on federal appointments.

Opening the Senate Judiciary Committee's first hearing on Obama's gun-control proposals on Wednesday, Chairman Patrick Leahy, D-Vt., implored his colleagues to move forward with Jones' nomination. But in an executive business meeting on Thursday, Grassley expressed reservations and criticized Jones for refusing to meet with his staff on the Fast and Furious investigation.

Grassley called the Oswald letter "disturbing." Among other allegations, the former Minneapolis FBI chief criticized Jones for helping advance the career of Assistant U.S. Attorney Carol Kayser, the daughter of one of Jones' mentors at the Minneapolis law firm of Robins, Kaplan, Miller & Ciresi, where Jones had been a partner.

Democrat contributor

Kayser's father, Thomas Kayser, is a major financial contributor to Democratic politicians, including Minnesota Sens. Amy Klobuchar and Al Franken, who sit on the Judiciary Committee.

Klobuchar and Franken have promised a full review of Jones' record. No date for a confirmation hearing has been set.

"Senator Klobuchar worked with Mr. Jones when she was Hennepin County attorney and found him to be a seasoned leader," Klobuchar spokeswoman Brigit Helgen said on Friday. "The senator and the Judiciary Committee will be taking a close look at materials submitted during the nominating process, where Mr. Jones will be allowed the opportunity to respond to any concerns during the committee's hearings."

Alexandra Fetissoff, a Franken spokeswoman, said that he "will review the information Senator Grassley receives in response to his inquiry, along with any additional information provided to the committee about Todd Jones, in preparation for Mr. Jones' confirmation hearing."

Kevin Diaz is a correspondent in the Star Tribune Washington Bureau.