Can Carl ever be President again? Interesting question. Here's the relevant wording (the rest is simply some legalese that says that it doesn't apply to Truman & it has to be ratified):
"Section 1. No person shall be elected to the office of the President more than twice, and no person who has held the office of President, or acted as President, for more than two years of a term to which some other person was elected President shall be elected to the office of the President more than once."
It is very short and to the point. Carl served more than two years of GWB's term, so he could only run for President one time. There are no exceptions for leaving office and then running again. He can't be elected President again, end of story.
However, I see no reason he couldn't run for another office and get there again, like he did before. The Presidential Succession Act of 1947 outlines who becomes President if the President dies. If he became a Vice President (#2) he could do it if the President died, which is how he got there in the first place. If he ran for Congress and made it to Speaker of the House, he would be #3 in line; figure a minimum of 10-12 years to make that happen. Next (#4) is the President pro tempore of the Senate, which would require Carl to run for the Senate & work his way up from there; traditionally this is the oldest Senator, usually in his 80s! Finally, if he was appointed to a Cabinet office (Secretary of State, Defense, etc.) he would be in the line of succession that way. He would simply need to bump off everybody in front of him and get away with it. Afterwards, he would be precluded from ever running for another term. Realistically, neither the VP slot or a cabinet post is possible - no President would want a subordinate around with Carl's track record to overshadow them. Sorry folks, but even Ernest Hemingway couldn't write that and make it believable! Carl is out of politics, except behind the scenes.
On a different note, watching the international news these days is very depressing. When I see the insanity that is happening in Iraq these days, I simply despair. Erbil, Mosul, Kirkuk, Sinjar - these are all places that in the Buckman universe turned out so different. I don't care whether you are Republican or Democrat, but the rule is "You break it, you bought it!" By God, but did we break it!