21:28 Nov 15, 2009 

Before posting the question, I had put "core question for the debate" (even though the "debate" isn't mentioned explicitly), but I worried that this didn't convey the idea of "background" -- I think you're solution is perfect! Thank you! ¡Saludos!

20:49 Nov 15, 2009 

The way I understand this, the "de fondo" expression in the original conveys both the "in the background" and "at the core" concepts, though I think "underlying" is much better for the first one (and it is shorter, too).