It is also a day to celebrate a common purpose, said Obama, who is making a rare joint appearance with Sen. John McCain, the GOP nominee, to show unity.  But Obama did not shrink from noting the prickly fact that hangs over every 9/11 anniversary this nation has observed:

Let us remember that the terrorists responsible for 9/11 are still at large, and must be brought to justice.

Obama has also used the attention on 9/11 to reiterate his foreign-policy differences with President George W. Bush and McCain. Responding to a question about Bush and former Mayor Rudy Giuliani, last night Obama told David Letterman:

I think Giuliani deserves credit.  He kept calm and he was clear.  You know, I still remember your show after it happened and how moving that was.  That was one of the most powerful moments on television.  I think that George Bush did the right thing by going after the Taliban in Afghanistan, and I would have done the exact same thing. And the big difference between myself and George Bush I think would have been to stay focused on Afghanistan — not get distracted by Iraq.

I think we would have tamped down Al Qaeda — we could have, if not captured or killed Bin Laden, at least made sure that they weren’t setting up the kind of base camps that have now reconstituted themselves, so they’ve got safe haven in the mountains between Pakistan and Afghanistan.

Sept. 11 is a day to “put aside politics,” says the conventional wisdom. We could all use a break from attack ads.

But it is also a day to embrace American values — which include democracy and robust debate. There may be no better day for Americans to hear out Obama’s foreign-policy arguments, and to consider whether Bush-McCain neo-conservatism has succeeded in catching bin Laden, or going on offense against the right targets. Or making the country safer.

The Democratic nominee, in answering a question about civil liberties from a member of the audience at a school gym, proactively raised Gov. Sarah Palin’s controversial attack from the Republican National Convention — when she claimed Democrats cared more about reading terrorists’ their rights than national security.

“First of all,” Obama said in his biting rebuttal, “you don’t even get to read them their rights until you catch them. They should spend more time trying to catch Osama bin Laden; and we can worry about the next steps later!”

As the crowd applauded, Obama punctuated his point with almost mock exasperation. “Seriously! These folks,” he said, turning to his own commitment to killing terrorists:

My position has always been clear: If you got a terrorist, take ‘em out — take ‘em out. Anybody who is involved in 9/11 — take ‘em out!

Yet unlike his Republican rivals, Obama argued that national security can be entirely consistent with upholding the Constitution:

But what have I also said is this: When you suspend habeas corpus, which has been a principal dating before even our county –- it’s the foundation of Anglo-American law — which says, very simply, if the government grabs you, then you have the right to at least ask, “Why was I grabbed” and say, “Maybe you got the wrong person.”

The reason you have that safeguard is because we don’t always have the right person. We may think this is Mohammed the terrorist, it might be Mohammed the cab driver.

Now I’ll shift from reporting to speculating, and note that in the moment Obama proffered the Mohammed example, he seemed to consider how the line might be misconstrued. Watching his voice rise in that packed Michigan gym, I think Obama knew he went farther than his typical line on racial profiling. (As you can see in the video below.)

To recalibrate, he gamely inserted himself into the habeas hypothetical. “You may think it’s Barack the bomb thrower, but it might be Barack the guy running for president.”

Then, Obama delivered a rousing close to his extemporaneous constitutional address:

So the reason that you have this principle is not to be soft on terrorism. It’s because that’s who we are – that’s what we’re protecting! (applause) Don’t mock the Constitution! (applause) Don’t make fun of it! Don’t suggest that it’s un-American to abide by what the founding fathers set up — it’s worked pretty well for over 200 years!

The crowd rose to its feet in loud applause. It was one of the most warmly received arguments of the day, a departure from the economic policies that anchor Obama’s current stump speech.

During the primaries, Obama’s habeas corpus line usually drew applause, and he has been advocating the issue in Congress.

Obama is one of the 13 senators to co-sponsor the Restoring the Constitution Act, a bill to restore the rule of law and roll back major parts of the Military Commissions Act, which the Supreme Court recently found unconstitutional. The 5-4 decision was praised by Obama, while Sen. John McCain derided it as one of the “worst” judgments in history.

Watching both parties’ conventions, you wouldn’t know the rule of law has been seriously undermined by the Bush administration, or that torture, rendition, domestic warrantless surveillance and other crimes have become common tools of U.S. policy under a corrupted Justice Dept.

It was heartening, if only for a few moments, to watch a citizen raise those issues, and for a presidential candidate to respond with principle and passion for restoring the constitutional order.