Should he zero in on Illinois' deep pool of political corruption with the resolve of his predecessor, who sent two governors to prison? Or should he devote even more resources to the gang- and drug-related violence that has claimed hundreds of lives, including in neighborhoods near President Barack Obama's own house?
The urgency of the question was highlighted by a weekend of violence that left seven people dead and more than three dozen wounded. But it's unclear what, if anything, U.S. attorneys can do to stem the bloodshed that has not already been tried.
The same question has arisen in other big cities, including New York and Los Angeles.
"Some think federal prosecutors can ride in on a white horse and end street crime. They can't," said Laurie Levenson, a former assistant U.S. attorney in Los Angeles. "The question is—how do you coordinate efforts of local and federal authorities? There's a role for both."
Other federal prosecutors have used the power of their office to attack urban crime. Rudy Giuliani became New York City mayor after first gaining prominence in the 1980s as a crime-busting U.S. attorney in Manhattan. And New Jersey Gov. Chris Christie built his political career on his reputation as a U.S. attorney who convicted public officials.
The pressure on Zachary Fardon has been especially intense.
After the White House recently named him to replace Patrick Fitzgerald, Democratic Sen. Dick Durbin—the Senate's second-most powerful member—and Republican Sen. Mark Kirk called on Fardon to target guns, gangs and drugs. Chicago Mayor Rahm Emanuel has also urged Fardon to make city violence a top priority.
The city's murder tally topped 500 in 2012, the first time since 2008 it hit that mark. Though the murder rate has declined in 2013, the killing early this year of 15-year-old honor student Hadiya Pendleton a mile from Obama's home put the issue back in the national news.
Kirk called on Fardon to use racketeering statutes to jail what he said were up to 18,000 Gangster Disciple members in the Chicago area, saying it would be "payback" for Hadiya's killing. Rep. Bobby Rush, a Chicago Democrat, dismissed the notion as a simplistic "white boy" solution to a complex problem.
But it's not as if the federal prosecutors didn't pursue violent crime during Fitzgerald's 11-year tenure.
Already, a third of the roughly 130 criminal prosecutors at the Chicago office are assigned to its gangs-and-drugs division, according to figures from office spokesman Randall Samborn. There are around 10 prosecutors in the public corruption and organized-crime division, though prosecutors often work together across divisions.
Despite the splash they make in the news, political corruption cases are relatively rare. Even at the height of the investigation surrounding disgraced former Gov. Rod Blagojevich, there were never more than a half a dozen politicians indicted a year.
Drug cases account for around a third of total prosecutions, according to data from the Administrative Office of the U.S. Courts for the year ending in September 2012. That number is mirrored in Chicago, Los Angeles and New York.
Federal prosecutors in Chicago have already used racketeering statutes to go after street gangs. Fitzgerald used them to send Latin Kings leader Augustin Zambrano to prison for 60 years last year. Nearly 30 gang members were convicted in the same investigation.
But drawing on federal drug or gun laws to go after gang members is more common because those laws can carry stiffer sentences and are often less labor-intensive than racketeering, or RICO, laws.
"RICO is often designed for the big hit, to take out the leadership of a gang and to send a larger message," Levenson said. "But RICO cases take a huge amount of resources and are far more complicated."
Observers have noted that Fitzgerald's success taking down heads of big street gangs had the unintended consequence of splintering gangs into dozens of factions, leading to fighting that has fueled some of the recent killing.
No matter what federal prosecutors do, city police and state prosecutors remain on the front lines of battle to reduce violence.
Just last week, local authorities took advantage of a new state racketeering law to arrest dozens of reputed leaders of the Black Souls street gang who allegedly engaged in beatings, kidnappings and killings to maintain their multimillion dollar narcotics operation. It's one of the first prosecutions using the new legal tool.
Fardon himself has been mum in public since his nomination last month. He has declined media interviews while awaiting a Senate confirmation vote, which hasn't been scheduled but should happen within a few months.
Tackling violent crime isn't what Fardon is best known for, at least up to now.
As an assistant U.S. attorney, he was a member of Fitzgerald's trial team that convicted former Republican Gov. George Ryan of corruption. And as a private defense attorney in Chicago in recent years, he focused in part on white-collar crime.
Fardon will also be under pressure not to scale back on the commitment to fighting corruption.
"The U.S. attorney's office, for all practical purposes, has been the sole check on corruption in Illinois," said Dick Simpson, a political scientist at the University of Illinois at Chicago. "And there is no indication corruption is on the decline."
But it could be difficult to shift more resources into violent crime without adjusting other obligations. That's especially true because congressional budget constraints have frozen staff at current levels. As a result, the annual office budget of around $35 million is unlikely to increase anytime soon.
The federal prosecutor's office in Chicago is one of the nation's busiest, with a staff already stretched thin handling more than 1,500 pending cases a year, according to the prosecutor's office.
And any hope of a dramatic reduction in violent crime would require the office to assign more prosecutors to smaller drug and gun cases.
"In a perfect world, you would grow the size of the office and hire more prosecutors and assign more to violent crimes units," said Julian Solotorovsky, a former assistant attorney in Chicago. "But the Department of Justice isn't in the hiring mood right now. ... That will complicate things for Fardon."
Follow Michael Tarm at www.twitter.com/mtarm