FOR SIX years President Bush has told Americans they face a “long war” against a global Islamic terrorist movement that, like the Cold War, will challenge a generation. A crucial if so far understated issue of the presidential campaign is whether that sweeping vision of U.S. national security will survive past January 2009. For the most part, the Republican candidates agree with Mr. Bush about the dimensions and centrality of the Islamic extremist threat. Most of the Democrats do not. From that ideological difference flow contrasting practical approaches to Iraq, Afghanistan and Iran, as well as differences in the weight the next president may give to other foreign policy challenges.