Like Philip supports and Mexico uses. Let's assume for simplicity's sake the Amendment establishing it is passed before 2000 so Bush becomes the first President under the new system. His term ends in 2006. Who would be the candidates and what would the map look like? I think it would be even worse for the GOP than the current map.
The nice thing about it would be that they wouldn't spend the first term running for re-election. Also, since the term would be lengthened by a factor of 50%, I think the legislative agenda could be more comprehensive. You should take it back to Carter, and assume his term ends in '82 instead of '80, and that there's no chance he could run again. My guess is that The Carter Doctrine wouldn't have evolved, and therefore The Bush Doctrine would not have been possible. On the down side, the double-digit inflation which gave rise to the rightward shift in national politics throughout the '80s would likely not have been ameliorated any sooner than it was, and the additional two years of malaise might have meant an even more radical legislative agenda than that which was pursued throughout the '80s.
On the other hand, I'm not a big fan of externally-imposed term limits in general. If the people want to continue to re-elect leaders (such as with Robert Byrd, Sam Rayburn, or Franklin Roosevelt), then I think that they should be allowed. In fact, much of the problems with extreme polarization and lack of centrists being elected from either party in various state legislatures stem from imposed term limits. But you're not talking about term limits for legislators, just for the President. I understand the difference, but I think that would necessarily mean that people would not have the liberty to re-elect a good leader during times of crises. As for lousy leaders, you could count on them not being re-elected at the end of their term so long as you have a well-informed public and fair elections. Still, lengthening the term from four to six years is still a good idea, with or without the one-term limit. You should start a petition.
Note that the sexenio has not been particularly good for mexico, and it did not prevent the PRI from a nearly 70-year stretch of one-party rule. I guess you could argue that the 20th century PRI was slightly less corrupt than the Porfiriato. Still, if you were going to promote a single term-limited presidency, you can find better examples.
