Associated Press Dennis Kucinich leaving Air Force One on Monday.

Quite a week for Dennis Kucinich, the liberal Democratic congressman who had been threatening to do to health care what Ralph Nader did to Al Gore in 2000.

On Monday, he accompanied President Obama on Air Force One, flying to his district in Cleveland for a rally.

On Wednesday, he ended months of self-righteous defiance, deciding not to stand with every single Republican in Congress trying to block health care reform.

Until the last minute, it looked like even an executive sky ride would not move the pure heart of Dennis Kucinich. When you’ve seen a U.F.O., as Kucinich says he has, a mere lobbying session at 32,000 feet by the Leader of the Free World, urging you to join your party in a cause that has eluded Democratic presidents since Franklin Roosevelt, is a tough match.

But let’s give him credit — he swallowed his pride and switched. Obama may not yet have the defining legislation of his presidency on his desk, but he’s already pulled off a small miracle: getting the holier-than-thou purists of his party to realize that they have to govern every now and then.

No such problems on the Republican side. Health care? It’s not even a debate point. They’re all for the status quo. In lock-step. Out in Arizona, where Senator John McCain is fending off a primary fight by the bombastic J.D. Hayworth, the obsession of the moment concerns whether gay marriage would allow a man to marry a horse — at least that’s how Hayworth sees it.

And even Republicans who had supported the philosophical framework of the plan now inching toward the finish line in Congress — that’s you, Mitt Romney, former Massachusetts governor and co-architect of the state’s near-identical health program — have fled to the thumb-sucking safety of the ideological playpen.

Ah, to be among the true believers, breathing only the clean air of sanctimony. Nothing is ever done, no lives improved, no laws passed. No messy deals tarnished by the poison of compromise. The public hates you — every poll shows that voters want both sides to legislate with a mix of ideas. But, oh, how good it must feel to be right all the time.

For most of his career, Kucinich has been a snow-white liberal. And it shows: after his nearly 14 years in Congress, his accomplishments could not fill the toe-end of a sock. Now, his vote could actually make a difference to the people in his congressional district who lack health care — about 90,000 or more if it follows the national average. They would have the right to buy the same coverage that Kucinich gets through his sweat-free job in Congress. Imagine.

“It’s not an easy burden he has taken up,” Kucinich said of Obama on Wednesday. “One of the things that has bothered me has been the attempt to delegitimize his presidency.”

No kidding. None of the great bipartisan triumphs of the past — Social Security, Medicare, the Civil Rights Act — would have a prayer in the present environment. That’s not how we do politics in 2010. We talk, loudly, only to like-minded partisans, and everyone else be damned.

If Kucinich had gone ahead as promised with a “no” vote, it would not have an asterisk next to it. It would simply be another no, putting him in league with Michele Bachmann, John Boehner and other congressional defenders of the costliest, most inefficient and least accessible health care system in the Western world.

Bachmann, who makes the nutty-news sidebar every time she opens her mouth, is the conservative doppelganger to the liberal purist. She can say crazy things — e.g., her claim that swine flu and Democratic presidents suggest a conspiracy of sorts — and actually get applause in her cocoon of kooks.

Reality is always a problem for purists. On the liberal side, many fail to comprehend that they are a distinct minority, stuck for years at around 19 percent of the public. When a liberal like Obama gets elected, he has to govern as a centrist for the simple reason that four-fifths of the country does not share his basic political outlook.

Smart liberals understand this. No one has been more passionate and consistent in favor of a single-payer universal health care system than Jim McDermott, the left-leaning congressman from Seattle.

But when the final health care tally is made, McDermott told me he plans to put aside his own concerns and vote for something less than perfect, rather than wait another 30 years.

“I always knew that we would need a fallback position,” he said, adding that he had “always considered myself a pragmatist to the core.”

These are easy votes for McDermott and Kucinich. To get into political trouble in their safe districts, they would have to do something like propose a national monument in honor of John Edwards.

Real courage is the first-term Democratic member of Congress in a Republican-leaning district who stands to lose the seat by voting for health care. There may be a dozen or more of those members who will walk the line by Saturday’s final vote.

A telling moment for Kucinich and Obama came during a presidential primary debate two years ago. After Kucinich admitted he had, in fact, seen a U.F.O. — it hovered silently overhead near the home of noted pragmatist Shirley MacLaine — Obama was asked if he believed in life on other planets.

He said he wasn’t sure, but what he did know was that there was plenty of life on this planet that needed help. Kucinich, in switching his vote, has for the moment decided to stop staring at saucers. Beam him up.