It is a incredibly successful political phrase,(meme/sound-bite), though; I find it hard to recall a broadcast vox pop in which it hasn't figured. I suppose it's strength and popularity relate to its broad-brush, almost vacuous appeal to those not willing or able to invest much thought into the decision before them. It also taps into officially sanctioned, permissible notions of patriotism...particularly to demographic cohorts that have heard first-hand from older relatives about their part in saving this country from foreign powers. For others I'd imagine that the word "back" offers another clue to the phrase's appeal to those nostalgic for 'warm beer, cricket and district nurses on bicycles'. Obviously for some it's a mantra that also allows for a politically accectable expression of xenophobia, if not outright racism.
Clever stuff.