"It is time to water the tree of liberty" said the sign carried by a gun-toting protester milling outside President Obama's town-hall meeting in New Hampshire two weeks ago. The Thomas Jefferson quote that inspired this message, of course, said nothing about water: "The tree of liberty must be refreshed from time to time with the blood of patriots and tyrants." That's the beauty of a gun - you don't have to spell out the "blood."

The protester was a nut. America has never had a shortage of them. But what's Tom Coburn's excuse? Coburn is a Republican senator from Oklahoma, where 168 people were murdered by right-wing psychopaths who bombed a federal building in Oklahoma City in 1995. Their leader, Timothy McVeigh, had the Jefferson quote on his T-shirt when he committed this act of mass murder. Yet last Sunday, when asked by David Gregory on "Meet the Press" if he was troubled by current threats of "violence against the government," Coburn blamed not the nuts but the government.

"Well, I'm troubled any time when we stop having confidence in our government," the senator said, "but we've earned it."

Coburn is nothing if not consistent. In the aftermath of the Oklahoma City bombing, he was part of a House contingent that helped delay and soften an antiterrorism bill. This cohort even tried to strip out a provision blocking domestic fund-raising by foreign terrorist organizations like Hamas. Why? The far right, in league with the National Rifle Association, was angry at the federal government for aggressively policing America's self-appointed militias. In a 1996 floor speech, Coburn conceded that "terrorism obviously poses a serious threat," but then went on to explain that the nation had worse threats to worry about: "There is a far greater fear that is present in this country, and that is fear of our own government." As his remarks on "Meet the Press" last week demonstrated, the subsequent intervention of 9/11 has not changed his worldview.

I have been writing about the simmering undertone of violence in our politics since October, when Sarah Palin, the vice-presidential candidate of a major political party, said nothing to condemn Obama haters shrieking "Treason!," "Terrorist!" and "Off with his head!" at her rallies. As vacation beckons, I'd like to drop the subject, but the atmosphere keeps getting darker.

Coburn's implicit rationalization for far-right fanatics bearing arms at presidential events - the government makes them do it! - cannot stand. He's not a radio or Fox News bloviator paid a fortune to be outrageous; he's a card-carrying member of the United States Senate. On Monday - the day after he gave a pass to those threatening violence - a dozen provocateurs with guns, at least two of them bearing assault weapons, showed up for Obama's V.F.W. speech in Phoenix. Within hours, another member of Congress - Phil Gingrey of Georgia - was telling Chris Matthews on MSNBC that as long as brandishing guns is legal, he, too, saw no reason to discourage Americans from showing up armed at public meetings.

In April the Department of Homeland Security issued a report, originally commissioned by the Bush administration, on the rising threat of violent right-wing extremism. It was ridiculed by conservatives, including the Republican chairman, Michael Steele, who called it "the height of insult." Since then, a neo-Nazi who subscribed to the anti-Obama "birther" movement has murdered a guard at the Holocaust museum in Washington, and an anti-abortion zealot has gunned down a doctor in a church in Wichita, Kan.

This month the Southern Poverty Law Center, the same organization that warned of the alarming rise in extremist groups before the Oklahoma City bombing, issued its own report. A federal law enforcement agent told the center that he hadn't seen growth this steep among such groups in 10 to 12 years. "All it's lacking is a spark," he said.

This uptick in the radical right predates the health care debate that is supposedly inspiring all the gun waving. Nor can this movement be attributed to a stepped-up attack by Democrats on this crowd's holy Second Amendment. Since taking office, Obama has disappointed gun-control advocates by relegating his campaign pledge to reinstate the ban on assault weapons to the down-low.

No, the biggest contributor to this resurgence of radicalism remains panic in some precincts about a new era of cultural and demographic change. As the sociologist Daniel Bell put it, "What the right as a whole fears is the erosion of its own social position, the collapse of its power, the increasing incomprehensibility of a world - now overwhelmingly technical and complex - that has changed so drastically within a lifetime."

Bell's analysis appeared in his essay "The Dispossessed," published in 1962, between John Kennedy's election and assassination. J.F.K., no more a leftist than Obama, was the first Roman Catholic in the White House and the tribune of a new liberal order. Bell could have also written his diagnosis in 1992, between Bill Clinton's election and the Oklahoma City bombing. Clinton, like Kennedy and Obama, brought liberals back into power after a conservative reign and represented a generational turnover that stoked the fears of the dispossessed.

While Bell's essay remains relevant in 2009, he could not have imagined in 1962 that major politicians, from a vice-presidential candidate down, would either enable or endorse a radical and armed fringe. Nor could he have imagined that so many conservative intellectuals would remain silent. William F. Buckley did make an effort to distance National Review from the John Birch Society. The only major conservative writer to repeatedly and forthrightly take on the radical right this year is David Frum. He ended a recent column for The Week, titled "The Reckless Right Courts Violence," with a plea that the president "be met and bested on the field of reason," not with guns.

Those on the right who defend the reckless radicals inevitably argue "The left does it too!" It's certainly true that both the left and the right traffic in bogus, Holocaust-trivializing Hitler analogies, and, yes, the protesters of the antiwar group Code Pink have disrupted Congressional hearings. But this is a false equivalence. Code Pink doesn't show up on Capitol Hill with firearms. And, as the 1960s historian Rick Perlstein pointed out on the Washington Post Web site last week, not a single Democratic politician endorsed the Weathermen in the Vietnam era.

This week the journalist Ronald Kessler's new behind-the-scenes account of presidential security, "In the President's Secret Service," rose to No. 3 on The Times nonfiction best-seller list. No wonder there's a lot of interest in the subject. We have no reason to believe that these hugely dedicated agents will fail us this time, even as threats against Obama, according to Kessler, are up 400 percent from those against his White House predecessor.

But as we learned in Oklahoma City 14 years ago - or at the well-protected Holocaust museum just over two months ago - this kind of irrational radicalism has a myriad of targets. And it is impervious to reason. Much as Coburn fought an antiterrorism bill after the carnage of Oklahoma City, so three men from Bagdad, Ariz., drove 2,500 miles in 1964 to testify against a bill tightening federal controls on firearms after the Kennedy assassination. As the historian Richard Hofstadter wrote in his own famous Kennedy-era essay, "The Paranoid Style in American Politics," these Arizona gun enthusiasts were convinced that the American government was being taken over by a "subversive power." Sound familiar?

Even now the radicals are taking a nonviolent toll on the Obama presidency. Obama complains, not without reason, that the news media, led by cable television, exaggerate the ruckus at health care events. But why does he exaggerate the legitimacy and clout of opposition members of Congress who, whether through silence or outright endorsement, are surrendering to the nuts? Even Charles Grassley, the supposedly adult Iowa Republican who is the Senate point man for his party on health care, has now capitulated to the armed fringe by publicly parroting their "pull the plug on grandma" fear-mongering.

For all the talk of Obama's declining poll numbers this summer, he towers over his opponents. In last week's Wall Street Journal-NBC News poll, only 21 percent approve of how Republicans in Congress are handling health care reform (as opposed to the president's 41 percent). Should Obama fail to deliver serious reform because his administration treats the pharmaceutical and insurance industries as deferentially as it has the banks, that would be shameful. Should he fail because he in any way catered to a decimated opposition party that has sunk and shrunk to its craziest common denominator, that would be ludicrous.

The G.O.P., whose ranks have now dwindled largely to whites in Dixie and the less-populated West, is not even a paper tiger - it's a paper muskrat. James Carville is correct when he says that if Republicans actually carried out their filibuster threats on health care, it would be a political bonanza for the Democrats.

In last year's campaign debates, Obama liked to cite his unlikely Senate friendship with Tom Coburn, of all people, as proof that he could work with his adversaries. If the president insists that enemies like this are his friends - and that the nuts they represent can be placated by reason - he will waste his opportunity to effect real change and have no one to blame but himself.