There was what President Obama likes to call a teachable moment last week, when the International Olympic Committee rejected Chicago’s bid to be host of the 2016 Summer Games.

“Cheers erupted” at the headquarters of the conservative Weekly Standard, according to a blog post by a member of the magazine’s staff, with the headline “Obama loses! Obama loses!” Rush Limbaugh declared himself “gleeful.” “World Rejects Obama,” gloated the Drudge Report. And so on.

So what did we learn from this moment? For one thing, we learned that the modern conservative movement, which dominates the modern Republican Party, has the emotional maturity of a bratty 13-year-old.

But more important, the episode illustrated an essential truth about the state of American politics: at this point, the guiding principle of one of our nation’s two great political parties is spite pure and simple. If Republicans think something might be good for the president, they’re against it — whether or not it’s good for America.

To be sure, while celebrating America’s rebuff by the Olympic Committee was puerile, it didn’t do any real harm. But the same principle of spite has determined Republican positions on more serious matters, with potentially serious consequences — in particular, in the debate over health care reform.

Now, it’s understandable that many Republicans oppose Democratic plans to extend insurance coverage — just as most Democrats opposed President Bush’s attempt to convert Social Security into a sort of giant 401(k). The two parties do, after all, have different philosophies about the appropriate role of government.

But the tactics of the two parties have been different. In 2005, when Democrats campaigned against Social Security privatization, their arguments were consistent with their underlying ideology: they argued that replacing guaranteed benefits with private accounts would expose retirees to too much risk.

The Republican campaign against health care reform, by contrast, has shown no such consistency. For the main G.O.P. line of attack is the claim — based mainly on lies about death panels and so on — that reform will undermine Medicare. And this line of attack is utterly at odds both with the party’s traditions and with what conservatives claim to believe.

Think about just how bizarre it is for Republicans to position themselves as the defenders of unrestricted Medicare spending. First of all, the modern G.O.P. considers itself the party of Ronald Reagan — and Reagan was a fierce opponent of Medicare’s creation, warning that it would destroy American freedom. (Honest.) In the 1990s, Newt Gingrich tried to force drastic cuts in Medicare financing. And in recent years, Republicans have repeatedly decried the growth in entitlement spending — growth that is largely driven by rising health care costs.

But the Obama administration’s plan to expand coverage relies in part on savings from Medicare. And since the G.O.P. opposes anything that might be good for Mr. Obama, it has become the passionate defender of ineffective medical procedures and overpayments to insurance companies.

How did one of our great political parties become so ruthless, so willing to embrace scorched-earth tactics even if so doing undermines the ability of any future administration to govern?

The key point is that ever since the Reagan years, the Republican Party has been dominated by radicals — ideologues and/or apparatchiks who, at a fundamental level, do not accept anyone else’s right to govern.

Anyone surprised by the venomous, over-the-top opposition to Mr. Obama must have forgotten the Clinton years. Remember when Rush Limbaugh suggested that Hillary Clinton was a party to murder? When Newt Gingrich shut down the federal government in an attempt to bully Bill Clinton into accepting those Medicare cuts? And let’s not even talk about the impeachment saga.

The only difference now is that the G.O.P. is in a weaker position, having lost control not just of Congress but, to a large extent, of the terms of debate. The public no longer buys conservative ideology the way it used to; the old attacks on Big Government and paeans to the magic of the marketplace have lost their resonance. Yet conservatives retain their belief that they, and only they, should govern.

The result has been a cynical, ends-justify-the-means approach. Hastening the day when the rightful governing party returns to power is all that matters, so the G.O.P. will seize any club at hand with which to beat the current administration.