A challenge too big? Wasn’t it conservatives who led that top to bottom butt kicking of the Democrats in 2010 when Republicans won back the House and were winning state and local elections? I realize those were congressional elections with a much lower turnout but the establishment wasn’t responsible for that awesome night back in November of 2010. With all due respect, I’m getting tired of this excuse that conservatives can’t topple the establishment. Conservatives are the only thing keeping the GOP afloat and yet they act as if they’re powerless.
Low-information left-wing voters don't bother turning out for off year elections, as you note.
They do turn out for Presidential elections. I assure you that the strategies and candidate types of 2010 may work for 2014, but they will fail in 2016.
With all that, Romney didn't face any serious competition in 2008, yet he lost to Huckabee and McCain, neither of who had any money or organization, for 2012, Romney had all the same, plus the next in line thing going for him, again no serious challenger ran against him.