Is he a viable candidate?
Sure. Why not? He has conventional qualifications, and is certainly in the mainstream of his party on public policy.
That said...Jamelle Bouie has been making the case that he's a strong candidate (at least on twitter; I don't think he's written it up elsewhere), and I'm not quite as impressed as he is. Mind you -- I was very bullish on Perry throughout the 2012 cycle, considering him a live longshot before he
So why not this time? Well, at least as of now, it appears that he'll be one of several viable candidates, rather than the one of two or three he was in summer 2011. Moreover, had Republican party actors wanted him in 2012, they almost certainly could have had him; debate debacles aside, a strong push for him in November through January by something like a unified group of GOP leaders would have very likely been enough.
And it's hard to believe that the reason that didn't happen was because of loyalty to, let alone enthusiasm for, Mitt Romney.
So the question becomes: if various flavors of Republican party actors weren't interested in 2012, against Romney, why would they be interested in 2016 against presumably three or four candidates who have better records of conservative orthodoxy than Romney could ever get?
That's not a rhetorical question. There may be an answer. Perhaps the debates really did scare people off who would be willing to jump on this time if Perry does well in the early debates. Perhaps Perry's deviations on abortion (the vaccine) and immigration, small as they seemed, mattered more to people than Romney's huge past-tense violations. Perhaps there really is something to the "wait for your turn" thing among Republicans. Perhaps Perry's health issues hurt his campaigning in general in ways that could be forgotten if he does well over the next couple of years.
But my guess is that while he's certainly viable, his failure in 2012 is telling us something about his 2016 candidacy, and that he's not all that likely to emerge from the pack this time around.