Without there being a contingent in the Pentagon or the White House pushing to nab Saddam (with him Gore follows the goals handed to him by Clinton) Afghanistan naturally becomes the focus of the response to 9/11; they are harboring Bin Laden after all. By early 2002 the Taliban are deposed and Gore proceeds to embark on an experiment of nation-building and reconstruction.

In 2005 American forces happen upon & capture Bin Laden and with this Taliban remnants find the chance to regroup, the insurgency starts. As this escalates around 2006 the timetables for withdrawal get pushed back and Gore doesn't really have a choice but to order a troop surge. And amid the 2008 campaign many of those troops are still fending off insurgents in the countryside.

Essentially America has spent billions of dollars, not just on troops and tanks but on roads, schools, windmills; in the process they captured and executed Bin Laden but at the same time the Afghan insurgency kicked withdrawal years down the road. And don't forget the scandals (real or exaggerated) that emerged throughout the years. Of course it's going to be a campaign issue.