First, most Americans will agree with him.

Al Qaeda tries to hide in Afghanistan, do we go after them?
Hell, yeah.
What if Pakistan complains?
Screw them.

Irresponsible?
Whatever.
Nobody cares about that.

The upshot is that Obama won't lose one vote because of that.

The left will figure he's better than McCain, at least... and 9/11 was pretty horrible.

The middle will agree that we should pursue al Qaeda, no matter what Pakistan says.

The right will think that maybe Obama's not quite as much of a wimp as previous Democratic candidates have been.

And so, he will actually gain votes.

One of the knocks on the Democrats in the past has been that they were wimpy.
9/11 is actually an opportunity, though, because there's finally a reason to be a hardass.
They attacked us, let's get them!

Obama is being careful to draw a distinction between fighting back against al Qaeda and attacking countries that have not actually attacked us.
This allows him to be kinda anti-war (Iraq war bad) while being strong on national defense (al Qaeda must be destroyed.)

This is a win-win for him.

The comment on Pakistan plays into this.

Americans will agree with the substance, and will mostly think better of him for it.

Bush is actually doing it, which only leaves the "he shouldn't have said it out loud" idea.
That won't play.
McCain thinks he's got a gotcha moment, but it's not going to work. The more he brings it up, the more middle America will say "hey, wait, I agree with that...!"

Note that Obama refuses to retract the statement.
And that attacking it isn't working for McCain.

Obama knows what he's doing.

Brilliant politics.