Republicans and Democrats don't agree on much, but they do agree on this: the middle class. At their conventions, the two parties will compete fiercely for its support.
Republicans will accuse Barack Obama of destroying the middle class through policies perpetuating high joblessness and feeble economic growth. Democrats will portray Mitt Romney as a tool of the rich who doesn't understand the middle class. To the victor may go the election, because "saving the middle class" has arguably become the campaign's defining issue.
This is mostly political symbolism. The idea that anyone can "save" the middle class assumes that it's in danger of disappearing, which it isn't, and that presidents possess sufficient powers to resurrect it, which they don't. Still, the symbolism is potent because most Americans equate the middle class with the kind of society we are and ought to be. It is a society where hard work and personal responsibility are rewarded — where "getting ahead" is expected; where economic security and social stability are enjoyed; and where privilege is minimized.