Hmmm. The Mars Society get enough members in a key state to be an important interest group to pander to and either Bush Junior or Gore makes them an important part of their election campaign. As OTL, the election is so tight people across the Atlantic can hear the squeak, and making good on the promises of the campaign and keeping the Mars Society onside for the next election are important enough that Zubrin is made NASA administrator and the president (whoever it ends up being) puts real political capital into getting a Mars-first space program soon.
Of course, that gets Zubrin into position and it gives him presidential support. But can he actually achieve his goals? Probably not in 8 years. And with the level of Congressional control over the NASA budget, he probably doesn't have enough money to achieve his goals, since NASA is starting underfunded and those funds are all already spoken for doing things in the states of a wide variety of Congresspersons who will fight to keep the Shuttle going, fight to keep NASA centers open, fight to keep the IS going, fight to keep developing rockets and hardware that they won't give NASA the money to use... Any change Zubrin wants to make is gonna be opposed by alot of people.
Whether he makes any progress toward his goals is a more interesting question. One of Zubrin's areas of interest was getting a SDHLV developed. If there's enough political backing and funding to properly fund that development (as opposed to trying to do it on the cheap and thus spending vastly more money than is needed as was done with the Ares V and SLS) Zubrin could perhaps help NASA avoid THAT debacle.
If 9/11 happens in this TL, it probably means bad things for the space program, just as Vietnam was very bad for NASA in the Apollo era.
fasquardon