God Bless America is a commonly uttered phrase by American politicians that is used as a subtle reminder to Americans who might have forgotten that, like any good parent, God picks favorites, and the favorite he picked is America.
The phrase was coined in the 1950s, during some of the most dangerous days of the Cold War. Many Americans feared that nucular war would annihilate the whole country. "God Bless America" was used to invoke God's blessings, some of which theoretically may have included some sort of immunity to nuclear warheads. For all we know, it was, and still is, 100% effective.
Republicans have free license to conclude speeches with the phrase, or use it to indicate to audience members when it is appropriate to wave their tiny plastic American flags; however, if Democrats use the phrase, it is because they are pandering and playing politics.
God Bless America is also the name of a very, very patriotic song that all Americans can sing without hesitation. Five times daily, while facing Houston, America's oil capitol.
Everyone except certain presidential candidates, apparently because it doesn't translate very well in Arabic.