But the laughs evaporated soon enough. There’s nothing entertaining about watching goons hurl venomous slurs at congressmen like the civil rights hero John Lewis and the openly gay Barney Frank. And as the week dragged on, and reports of death threats and vandalism stretched from Arizona to Kansas to upstate New York, the F.B.I. and the local police had to get into the act to protect members of Congress and their families.

How curious that a mob fond of likening President Obama to Hitler knows so little about history that it doesn’t recognize its own small-scale mimicry of Kristallnacht. The weapon of choice for vigilante violence at Congressional offices has been a brick hurled through a window. So far.

No less curious is how disproportionate this red-hot anger is to its proximate cause. The historic Obama-Pelosi health care victory is a big deal, all right, so much so it doesn’t need Joe Biden’s adjective to hype it. But the bill does not erect a huge New Deal-Great Society-style government program. In lieu of a public option, it delivers 32 million newly insured Americans to private insurers. As no less a conservative authority than The Wall Street Journal editorial page observed last week, the bill’s prototype is the health care legislation Mitt Romney signed into law in Massachusetts. It contains what used to be considered Republican ideas.

Yet it’s this bill that inspired G.O.P. congressmen on the House floor to egg on disruptive protesters even as they were being evicted from the gallery by the Capitol Police last Sunday. It’s this bill that prompted a congressman to shout “baby killer” at Bart Stupak, a staunch anti-abortion Democrat. It’s this bill that drove a demonstrator to spit on Emanuel Cleaver, a black representative from Missouri. And it’s this “middle-of-the-road” bill, as Obama accurately calls it, that has incited an unglued firestorm of homicidal rhetoric, from “Kill the bill!” to Sarah Palin’s cry for her followers to “reload.” At least four of the House members hit with death threats or vandalism are among the 20 political targets Palin marks with rifle crosshairs on a map on her Facebook page.

When Social Security was passed by Congress in 1935 and Medicare in 1965, there was indeed heated opposition. As Dana Milbank wrote in The Washington Post, Alf Landon built his catastrophic 1936 presidential campaign on a call for repealing Social Security. (Democrats can only pray that the G.O.P. will “go for it” again in 2010, as Obama goaded them on Thursday, and keep demanding repeal of a bill that by September will shower benefits on the elderly and children alike.) When L.B.J. scored his Medicare coup, there were the inevitable cries of “socialism” along with ultimately empty rumblings of a boycott from the American Medical Association.

But there was nothing like this. To find a prototype for the overheated reaction to the health care bill, you have to look a year before Medicare, to the Civil Rights Act of 1964. Both laws passed by similar majorities in Congress; the Civil Rights Act received even more votes in the Senate (73) than Medicare (70). But it was only the civil rights bill that made some Americans run off the rails. That’s because it was the one that signaled an inexorable and immutable change in the very identity of America, not just its governance.

The apocalyptic predictions then, like those about health care now, were all framed in constitutional pieties, of course. Barry Goldwater, running for president in ’64, drew on the counsel of two young legal allies, William Rehnquist and Robert Bork, to characterize the bill as a “threat to the very essence of our basic system” and a “usurpation” of states’ rights that “would force you to admit drunks, a known murderer or an insane person into your place of business.” Richard Russell, the segregationist Democratic senator from Georgia, said the bill “would destroy the free enterprise system.” David Lawrence, a widely syndicated conservative columnist, bemoaned the establishment of “a federal dictatorship.” Meanwhile, three civil rights workers were murdered in Philadelphia, Miss.

That a tsunami of anger is gathering today is illogical, given that what the right calls “Obamacare” is less provocative than either the Civil Rights Act of 1964 or Medicare, an epic entitlement that actually did precipitate a government takeover of a sizable chunk of American health care. But the explanation is plain: the health care bill is not the main source of this anger and never has been. It’s merely a handy excuse. The real source of the over-the-top rage of 2010 is the same kind of national existential reordering that roiled America in 1964.

In fact, the current surge of anger — and the accompanying rise in right-wing extremism — predates the entire health care debate. The first signs were the shrieks of “traitor” and “off with his head” at Palin rallies as Obama’s election became more likely in October 2008. Those passions have spiraled ever since — from Gov. Rick Perry’s kowtowing to secessionists at a Tea Party rally in Texas to the gratuitous brandishing of assault weapons at Obama health care rallies last summer to “You lie!” piercing the president’s address to Congress last fall like an ominous shot.

If Obama’s first legislative priority had been immigration or financial reform or climate change, we would have seen the same trajectory. The conjunction of a black president and a female speaker of the House — topped off by a wise Latina on the Supreme Court and a powerful gay Congressional committee chairman — would sow fears of disenfranchisement among a dwindling and threatened minority in the country no matter what policies were in play. It’s not happenstance that Frank, Lewis and Cleaver — none of them major Democratic players in the health care push — received a major share of last weekend’s abuse. When you hear demonstrators chant the slogan “Take our country back!,” these are the people they want to take the country back from.

They can’t. Demographics are avatars of a change bigger than any bill contemplated by Obama or Congress. The week before the health care vote, The Times reported that births to Asian, black and Hispanic women accounted for 48 percent of all births in America in the 12 months ending in July 2008. By 2012, the next presidential election year, non-Hispanic white births will be in the minority. The Tea Party movement is virtually all white. The Republicans haven’t had a single African-American in the Senate or the House since 2003 and have had only three in total since 1935. Their anxieties about a rapidly changing America are well-grounded.

If Congressional Republicans want to maintain a politburo-like homogeneity in opposition to the Democrats, that’s their right. If they want to replay the petulant Gingrich government shutdown of 1995 by boycotting hearings and, as John McCain has vowed, refusing to cooperate on any legislation, that’s their right too (and a political gift to the Democrats). But they can’t emulate the 1995 G.O.P. by remaining silent as mass hysteria, some of it encompassing armed militias, runs amok in their own precincts. We know the end of that story. And they can’t pretend that we’re talking about “isolated incidents” or a “fringe” utterly divorced from the G.O.P. A Quinnipiac poll last week found that 74 percent of Tea Party members identify themselves as Republicans or Republican-leaning independents, while only 16 percent are aligned with Democrats.

After the Civil Rights Act of 1964 was passed, some responsible leaders in both parties spoke out to try to put a lid on the resistance and violence. The arch-segregationist Russell of Georgia, concerned about what might happen in his own backyard, declared flatly that the law is “now on the books.” Yet no Republican or conservative leader of stature has taken on Palin, Perry, Boehner or any of the others who have been stoking these fires for a good 17 months now. Last week McCain even endorsed Palin’s “reload” rhetoric.

Are these politicians so frightened of offending anyone in the Tea Party-Glenn Beck base that they would rather fall silent than call out its extremist elements and their enablers? Seemingly so, and if G.O.P. leaders of all stripes, from Romney to Mitch McConnell to Olympia Snowe to Lindsey Graham, are afraid of these forces, that’s the strongest possible indicator that the rest of us have reason to fear them too.

•

Correction: Timothy Geithner’s title at the Federal Reserve Bank of New York was president and chief executive officer, not chairman, as I wrote here last week.