From a reader:

Jonah,
A simple Google search (Saddam September 11 poll)
yields a number of poll results showing majorities
believing there to be a connection. The fifth result
is a WAPO poll showing 69% believing it was likely on
or around 9/6/03.
The first result, however, is a Harris interactive
poll from February of this year showing only, “47
percent believe that Saddam Hussein helped plan and
support the hijackers who attacked the U.S. on
September 11, 2001 (up six percentage points from
November).”
A fair read would suggest that the american public has
been slowing disabusing themselves of this once fairly
widely held belief.

Me: Okay, so what poll is Gore referring to in the second part of this statement?:

I thought maybe it was an aberration when three-quarters of Americans said they believed that Saddam Hussein was responsible for attacking us on September 11, 2001. But more than four years later, between a third and a half still believe Saddam was personally responsible for planning and supporting the attack.