The Southern Poverty Law Center reported the rising numbers on Tuesday in its annual report on extremist groups.
The number of anti-government patriot groups, one category tracked by the center, rose dramatically over the past four years, from 149 groups in 2008 to 1,360 today, researchers reported. That was up about 7 percent from the 1,274 active in 2011.
The election and re-election of the nation's first black president and the rugged economy have fueled their growth, said Mark Potok, a senior fellow at the SPLC.
"The anger, angst, frustration, fear surrounding the economy have very much poured fuel on this fire," Potok said.
On gun control, the debate following the Newtown, Conn., mass murder of schoolchildren has led to "a kind of white-hot rage unleashed on both the radical right and also within more mainstream political circles," he said.
In the week following the Dec. 14 school shooting, the Federal Bureau of Investigation said it conducted more background checks for firearms sales and permits to carry than it has in any other one-week period since 1998.
Some critics believe the SPLC is too broad in labeling hate and extremist groups, to the point of including legitimate political organizations that oppose illegal immigration, gun control, gay rights and other issues.
The center's researchers say they use a variety of methods to track anti-government groups and compile their list from field reports, patriot publications, law enforcement sources and news reports. Potok said only active groups are included.
"We are not just looking at one man and a computer," he said.
The report states: "Generally, Patriot groups define themselves as opposed to the 'New World Order,' engage in groundless conspiracy theorizing, or advocate or adhere to extreme anti-government doctrines."
The FBI defines military extremists as anti-government groups often organized into paramilitary groups that follow a military-style rank hierarchy and typically engage in wilderness, survival, or other paramilitary training, according to a September 2011 FBI report on domestic terrorism.
Along with the rise of extremist groups, Potok said there have been several home-grown terrorist plots against government buildings and leaders in recent months. He compared the climate to time leading up to the 1995 Oklahoma City bombing.
The report cites several cases, including one centered on a Georgia Army base involving a group known as F.E.A.R. (Forever Enduring Always Ready). Federal prosecutors maintain that F.E.A.R. was led by active-duty soldiers at Ft. Stewart who also plotted bomb attacks in Savannah and aimed to poison apple crops in Washington state.