He's the boss, right? So he decided:

1. We will develop and test a crew exploration vehicle (CEV) by 2008.
2. We will finish the space station (ISS) by 2010.
3. We will focus on biology of human endurance in microgravity, at the cost of other ISS science.
3. We will retire the space shuttle (Orbiter) by 2010.
4. We will send more rovers and probes to the Moon and Mars to decide the best place to land humans.
5. We will land people on the Moon by 2014 using the CEV.
6. We will have a manned lunar base on the Moon by 2020.
7. We will THEN work on sending people to Mars.

He gave NASA a presidential order, including telling them how to spend their money. So this is the order in which things are going to occur, unless future presidents purposely cancel the plan. There is no more discussion about what's going to happen first. Even if eventually it were proven "Mars first" were a marginally better plan, right now we don't know. So Bush effectively tossed a coin, and decided that we're sending astronauts to the Moon first!

In the near term, Bush might get re-elected, or not, and maybe even serve more terms in the future. How many US presidents have served more than two terms? I'm not in the know about that. As I understand it even if he is voted out, his replacement would have to purposely cancel the plan. His replacement might not care, because the funding Bush gave NASA is not significantly higher than NASA already had. So as long as NASA is left alone in the future, and does not have cost overruns, the Return to the Moon plan will carry forward.

And Bush literally doesn't have to pay attention to it, unless it pays off politically.