The nonprofit civil rights organization, which tracks the hate movement and anti-government groups, counted 512 militias and related groups in 2009, up from 149 groups the year before. And, it said, the movement has added a layer of racism largely absent a decade ago.
At the same time, the organization has observed a spike in what it calls "nativist extremist groups," defined as those that "confront or harass suspected immigrants," to 309 groups last year, up from 173 the year before.
Fear and frustration were the fuel, the report said.
"The anger seething across the American political landscape ... goes beyond the radical right," the report said, adding that the rage was fed by "racial changes in the population, soaring public debt and the terrible economy, the bailouts of bankers and other elites, and an array of initiatives by the relatively liberal Obama administration that are seen as 'socialist' or even 'fascist.'"
"The 'tea parties' and similar groups that have sprung up in recent months cannot fairly be considered extremist groups," the report said, "but they are shot through with rich veins of radical ideas, conspiracy theories and racism."
And some of the underlying beliefs of the militia movement have found their way into the mainstream, according to the report, "Rage on the Right: The Year in Hate and Extremism," which was written by Mark Potok, director of the SPLC's Intelligence Report.
"While in the 1990s, the movement got good reviews from a few lawmakers and talk-radio hosts, some of its central ideas today are being plugged by people with far larger audiences, like Fox News' Glenn Beck and U.S. Rep. Michele Bachmann," Potok wrote. "Beck, for instance, re-popularized a key Patriot conspiracy theory -- the charge that FEMA is secretly running concentration camps -- before finally 'debunking' it."
The SPLC last year reported on the resurging militia movement, which it said was propelled by conspiracy theories about pending martial law and a move by Mexico to reclaim portions of the American Southwest. With the election of Barack Obama as president, that report said, the new wave of militia activity had taken on a much more racist cast than the movement that gave rise to Timothy McVeigh's bombing of the Alfred P. Murrah Building in Oklahoma City on April 19, 1995.
The SPLC's new assessment comes less than two weeks after Joseph Stack became a hero to some anti-government crusaders when he crashed a small airplane into an Internal Revenue Service office in Austin, Texas, killing himself and an IRS employee. Stack had posted an online "manifesto" detailing his financial problems and frustration with the federal government, and said he intended to give the IRS "my pound of flesh."
Experts are already concerned that Stack's suicidal plane crash could make him a martyr to anti-government zealots, and, like the deadly federal raid on David Koresh's Branch Davidian Ranch in Waco, Texas, and the violent siege of Randy Weaver's cabin in remote Ruby Ridge, Idaho, could spawn a fresh wave of right-wing violence.
The SPLC said it already has noticed "signs of similar violence," including the killings of six law enforcement officers, arrests of extremists in alleged assassination plots against Obama, a shooting rampage by a white supremacist in Brockton, Mass., and the arrests of "individuals with anti-government, survivalist or racist views ... in a series of bomb cases."