Doesn't look like he's declining to me:
His poll numbers aren't. That is a big lie people are perpetrating based on a couple fishy results directly after debate number #2
However, his favorabilities are basically getting crushed and that should be a concern because he won't be winning head-to-heads with other candidates (and he was never leading Carson H2H)
Still, Trump leads. Still, Trump's the frontrunner. Carly Fiorina has declined since jumping into double digits at one point and no one starts posts of "Why is Carly Fiorina declining in the polls?"
Trump's the leader. Carson is the second big outsider. The problem for the Establishment is selling enough of the Trump-Carson fans on a candidate who's more of the same old crap.