I do wonder what kind of non-establishment challengers we'd see. Would the most significant among them be traditional liberals (I'm struggling to think of a realistic example), or would they be of a different flavor (e.g. Schweitzer)?
The template I would use is Gore vs. Bradley 2000, where the frontrunner (Gore in 2000; Clinton in 2016) is such an imposing frontrunner that he/she only gets one challenger of any consequence, who looks like he might have a chance at making something happen in either IA or NH early on, but ends up losing all 50 states to the frontrunner in the end. The challenger is someone who goes after the frontrunner from the left, and who's at a stage in his career where he doesn't really have anything to lose by running. (Don't know who that would be in 2016, but an obvious possibility would be Feingold.)
My reasoning for why Clinton would be so much stronger in 2016 than she was in 2008 that she'd clear the field:
1) She's the most popular politician in the country at the moment. And that's with the general electorate. Among Democrats, she has a phenomenal 86% favorability rating:
http://www.publicpolicypolling.com/pdf/2011/PPP_Release_National_417.pdfIt's quite rare for candidates to begin a presidential campaign with numbers like that. Unless she does something to screw that up in her last year as SoS, she'll probably keep those numbers through 2014 and into 2015 when candidates have to start declaring for the 2016 primaries.
2) In 2006/7, there were quite a few qualms about HRC among pragmatic Dems who just wanted to win. (Many establishment Dems were urging Bayh and Warner to run, because they thought HRC was a sure loser.) She was seen as a polarizing figure who might not be able to win a general election. Those concerns have now evaporated because of reason #1. For that reason, I don't see any establishment competition for her in 2016.
3) In 2006/7, there was also quite a bit of suspicion and even outright hostility towards her from the liberal base of the party, because of her triangulation on various issues as a senator, and especially about the Iraq War vote. The Iraq War has faded as an issue now, and I doubt that'll be much of a factor in 2016. Also, many in the liberal base have been disillusioned by Obama anyway, so they'll probably end up just being more apathetic in 2016, not as easily able to get excited about a challenger to Clinton.
4) Among some Dems, there's a sense of "unfinished business" from 2008 about electing the first female president, and Clinton is the most obvious vehicle through which to do it in 2016.
5) People say that only the GOP ever nominates people who've run before, but it's rare that the runner up on the Democratic side comes as close to winning as Clinton did in 2008. The closest parallel would be Gary Hart in 1984, who would have actually been the prohibitive frontrunner in 1988 if he hadn't gotten himself in trouble re: Donna Rice. As long as Hillary doesn't have an affair with Clarence or something, she should be OK.
