There will come a moment - fairly soon - when Bush will cease to be the standard bearer of the Republican party. Various competing factions will be going for the presidential nomination and while Bush's implicit or explicit endorsement might be helpful in the primaries I highly doubt that Bush will be a big help to whoever his potential successor is in the general election campaign.
The fascinating thing will be watching how the Republican Party unwinds itself from the George Bush personality cult it's become. The truth is, no matter what they pretend to think, they don't really like him. While the obedience and deference to Bush has far exceeded that which Reagan got during his time in office, it's impossible to imagine that Bush will replace Reagan as the patron saint of the party.
So we'll be watching Republicans as they slowly abandon their hero at which point he'll basically be forgotten.
One thing would disrupt this storyline - Jeb.
