Arguably the most politically significant aspect to Tuesday's Senate testimony from Gen. David Petraeus and ambassador Ryan Crocker is the campaign angle -- John McCain, Hillary Clinton and Barack Obama will all get a chance to question Petraeus and Crocker directly.

But McCain is drawing interest this morning for what appears to be yet another in a series of mistakes about the basics in the Middle East.

For those who can't watch clips online, McCain asks Petraeus, "Do you still view al-Qaida in Iraq as a major threat?" The general responded, "It is still a major threat, though it is certainly not as major a threat as it was, say, 15 months ago." McCain said, "Certainly not an obscure sect of the Shiites all overall?" Petraeus answered, "No," and McCain quickly added, "Or Sunnis or anybody else."

I've watched the exchange a few times, and I keep coming to the same conclusion: By rhetorically asking if al-Qaida is a Shiite sect, McCain was once again demonstrating that he's confused about the terrorist group's religious background. He added, "Or Sunnis or anybody else" not to necessarily to clarify but to cover his bases -- he figures al-Qaida has to be affiliated with an Islamic tradition, even if he doesn't know which one.

Ilan Goldenberg added, "McCain did genuinely mix up Sunnis and Shi'a again ... Now, I know that there is a bit of gotcha going on here. But this man claims that his greatest qualification for the Presidency is that he understands foreign policy. But the differences between Sunni and Shi'a matter. They matter a lot! And this nasty habit of mixing it up just seriously needs to stop."

Indeed, I'd say it's the "nasty habit" that makes this morning's mix-up especially interesting. If McCain had consistently demonstrated a firm grasp of events in the Middle East, it'd be easier to overlook confusion over whether al-Qaida is Sunni or Shiite.

But therein lies the point. McCain has struggled with the basics more than once recently.

Add up his errors, and we see a Republican candidate whose problem is not with words but with facts.