So, Bernie Sanders has started to get an increased amount of media attention lately. Take that with a grain of salt; part of it comes from the fact that his campaign has been doing some interesting things, and part of it is due to the media's need to cover a second (or more) viable Democratic candidate so that the nomination process is something more exciting than a Clinton Coronation.
The second caveat is that Sanders has been harvesting some low hanging fruit. Let's put it this way: if you're the progressive candidate and you can't get 10,000 people to come out to see you in Madison, WI you might as well throw in the towel. The headlines about his early draw in places like Oregon, Madison, and Berkeley are up there with "Dog licks balls" as riveting news. Furthermore, a good deal of the national media attention he has received is condescending, depicting him as your burned out old hippie uncle who once lived on a commune and probably has an extremely high electric bill (wink). It is hardly as if full scale BernieMania is on the verge of sweeping the country.
That said, he has a real shot at this. And Team Clinton must be shitting bricks right now. I suspect that more than a few who were around her toxic campaign in 2008 dare not say "Uh oh. Here we go again…" even though they're thinking it.
Here is the problem from Clinton's perspective. As commanding as her lead in name recognition and money over the rest of the Democratic "field" is, 75% of the party base is looking around hoping someone better will come along. Hillary Clinton seems like she could win a general election, and therefore nearly all left-of-center Americans consider her Acceptable. Acceptable and Great are not quite the same thing, however. She is the classic establishment candidate, and her argument for the nomination boils down to "It's my turn, and I deserve this." Like other establishment candidates (Mitt Romney, for example, who also saw his party desperately try to nominate literally anyone but him before circling back) her primary motivation seems to be that she really, really wants to be president. That's fine, but you can't make it this obvious. Democratic voters are more susceptible to idealists than Republicans. Someone who can come along and convince the party base that he or she might make things better rather than merely being Slightly Better than the Republican.
In 2008, Obama came out of nowhere, offered this to primary voters, and immediately eliminated Clinton's insurmountable lead. Sanders is capable of doing that. And I suspect the Clinton campaign knows it. Even if they won't say it, I think they also recognize that the fundamental problem is that while voters will take Clinton, nobody's terribly enthusiastic about it. We're all scanning the horizon looking for someone less infuriatingly Centrist, less New Democrat, less I Refuse to Take Positions on Anything of Importance and I'm Basically John McCain on Foreign Policy. Again, in a general election Democrats and left-leaning voters will vote for her overwhelmingly. She's good enough. Clearing the bar just barely will always leave her susceptible to challengers who leap over it and look like they could actually win a general election.
Sanders is not without flaws himself. He needs to be more conscious of his visuals when on stage or behind a podium. He needs some people who have a proven track record of running a winning campaign. He needs a goddamn haircut. As he stands, any opponent would have a very easy time labeling him an old codger from a bygone era. Image counts. Hillary Clinton (67) is nearly as old as Sanders (73) but looks nowhere near that age. Most people would be surprised to learn that she is even 60. Sanders also needs to avoid the fate of Howard Dean. The media will be eager to depict him as an old, ranting lunatic from the hinterlands and he needs to be careful not to give them the opportunity.
In 2016 the Democrats have a chance to give themselves a real leg up by the time the general election begins. While the GOP 20-way circus plays out as monkeys in suits flinging their own crap at one another on debate stages, the two older, mature looking and sounding Democratic candidates will turn on how well Clinton can convince voters that she is something better than Republican Lite and how "electable" Sanders can make himself sound.
In short, greet Sanders' increased visibility with guarded optimism. He can win, even if the odds remain a long shot for now. The best evidence that he is viable is the Clinton campaign's sense of alarm as they realize that she is not in fact the candidate of inevitability.