I think you're right about Zarqawi being the final straw, this has been building up since the first Iraqi election. Things have been progressively getting better since then and I believe they've run out of phony excuses. They can say things are getting worse when in actuality they are getting better only so long.
I think Zarqawi biting the big one, on the heels of the special election in California to replace Duke Cunningham, AND no Rove indictment just sent the RATS completely over the edge. That's three huge whammies in less than a week, and being as they're already unbalanced, this triple play just sent them right where they were already headed. This not only took immigration off the front pages, but it revitalized a demoralized GOP and its base. Finally, the whole William Jefferson fiasco didn't help them.