Founded on the heels of America's civil rights movement, the Southern Poverty Law Center burnished its name with legal wins over white-supremist groups and helped integrate institutions like the Alabama State Police.
But it has also stoked anger with the Family Research Council, a conservative Christian policy group that lashed out this week after a 28-year-old gunman stormed their downtown Washington headquarters and opened fire.
"Let me be clear that Floyd Corkins was responsible for firing the shot yesterday," said council president Tony Perkins. "But Corkins was given a license to shoot an unarmed man by organizations like the Southern Poverty Law Center that have been reckless in labeling organizations hate groups because they disagree with them on public policy."
The SPLC has listed The Family Research Council as a hate group since 2010, pointing to what it describes as its anti-gay propaganda and legislative agenda.
In 1999, an FRC analyst co-authored a document titled "Homosexual Activists Work to Normalize Sex With Boys," according to the law center.
The booklet, which is not available on the FRC website, reportedly argued that "the primary goals of the homosexual rights movement is to abolish all age of consent laws and to eventually recognize pedophiles as the 'prophets' of a new sexual order," according to the SPLC.
The group opposes same-sex marriage and lobbied to prevent the repeal of the "don't ask, don't tell" policy, which prohibited military service by openly gay men and women.
Perkins said the SPLC "should be held accountable for their reckless use of terminology that is leading to the intimidation and what the FBI here has categorized as an act of domestic terrorism."
The gunman apparently said, "I don't like your politics" during the incident, and had been carrying 15 Chick-fil-A sandwiches, a possible connection to the restaurant chain's CEO and his stance against gay marriage.
Corkins, a former George Mason University student, had reportedly worked as a volunteer at an organization serving the lesbian, gay, bisexual and transgender communities.
He allegedly shot a security guard on Wednesday at the building before being wrestled to the ground.
But soon after Perkins' comments, the SPLC defended its classification of the council.
"Perkins' accusation is outrageous," said Mark Potok, a SPLC Senior Fellow.
"The FRC and its allies on the religious right are saying, in effect, that offering legitimate and fact-based criticism in a democratic society is tantamount to suggesting that the objects of criticism should be the targets of criminal violence."
While the SPLC says it distinguishes groups that spread hateful information from violent ones, its final tally, according to Booth Gunter, a SPLC spokesman, largely depends on the answer to a single question: Does a group promote hate?
"It's not an estimate of criminality," Gunter said. "Some are violent and some aren't."
Earlier this month, the Montgomery-based SPLC came under scrutiny after it announced that it had monitored Wade Michael Page, a 40-year-old gunman and former front-man for a white supremacist rock band.
Page killed himself in a suburban Milwaukee parking lot on August 5 after fatally shooting six people in a Sikh temple, less than two weeks after a deadly shooting at a movieplex in Aurora, Colorado.
"There are thousands of guys like that," said Gunter. "Unless we get some information that he's planning an attack, we don't usually call law enforcement."
The group instead monitors and publishes material about extremist groups, and offers police training, ranging from how to interpret tattoos to what officers should be aware of during traffic stops.
Speckled across the SPLC's online map is a hive of red numbers that reveals the nation's "hate groups."
Information on skinheads, neo-Nazis and Klan members comprise the thousand-plus organizations suspected of spreading hate.
Each name is accompanied by a symbol, with characters ranging from a crescent moon, signifying anti-Muslim, to a fist, which points to white nationalism.
The privately funded SPLC says it is nonpartisan, funded by member donations and a $223 million endowment. But it has been criticized by groups on the right for leaning left.
It was incorporated in 1971, with civil rights activist Julian Bond named as its first president.