I have no objection to the possibility of Bernie Sanders winning the Democratic nomination. But a lot of people do. Since this is all hypothetical and we haven't even gotten to Iowa yet, it feels appropriate to do a thought exercise of how to stop Sanders *if* he starts winning *and* you are of the mindset that that is a bad thing.
Sanders won NH in 2016 and figures to win it again this time. Joe Biden is likely to walk away with SC, the third contest on the calendar. Sanders currently leads in Iowa but the poll data for Iowa has been all over the place; look around and you can find any of four different candidates in a narrow lead. South Carolina notwithstanding, there will definitely be a little bit of trepidation in Never Sanders camps if he were to win Iowa and NH.
The Democrats' options in that scenario would require something they're bad at – decisive, coordinated action as a party – to avoid falling into the same trap that the Republicans fell for in 2016 when someone from outside their party came in and took the nomination. Were I being paid to give my advice, which of course I am not, here's how I'd lay out the options:
1. Do nothing. It's possible, in a Sanders winning IA and NH scenario, to write it off as a fluke from two unrepresentative states. "So what, winning Iowa doesn't mean anything." Wait and hope the Sanders campaign runs out of whatever gas it has over the past few weeks, and hope one of the other more mainstream candidates heats up and starts running off wins. This would involve two risks. One, that Sanders might keep winning. Two, that the large pool of other candidates would not split the wins in other states and allow Sanders to finish as a plurality winner of delegates, not a majority.
2. Rally to one candidate. The fundamental problem the GOP had in 2016 was that while the majority of primary voters were not voting for Trump, they could not settle on one "not Trump" from a particularly bad set of choices including Jeb Bush, Ted Cruz, and John Kasich. Had the party reacted to the first couple Trump plurality wins by immediately throwing everything behind one other candidate they would have stopped him.
The Democratic Party organization would have to take the two most viable campaigns – Biden and Warren – and make a hard but final choice of which one to run with. Big donors, party elites, media personalities, and voters alike would have to come to one consensus choice. Biden would make his "electability" argument and Warren would argue that a wider swath of the party supports her. Since there is no mechanism for a party to force candidates to withdraw, the Democratic Party would have to do a thing it really dislikes doing: politics. How do you convince one to drop out? I don't know. Use your imagination. Sit everyone down, lay out the stakes, and find a way to make one of the campaigns agree to fold in exchange for something they want. Easy? No. Realistic? Barely. Possible? Yes. "Dire times call for dire reactions." Have Obama and the Clintons involved internally. Do whatever is necessary to come to a consensus, however bitter.
The other, smaller campaigns – Klobuchar, etc. – have to be told in no uncertain terms that they are personae non grata in the party if they don't take the hint and fold up after a string of losses. Give a lot of "Be a good soldier" talks and hope it sticks.
3. Mount (or keep mounting) an anti-Sanders campaign among prominent Democrats – This seems to be the current strategy and I don't think there's any evidence it's a good strategy. With an outsider candidate who defines himself as "not of the Establishment," attacks from The Establishment will help him far more than they will hurt him. I am not kidding when I say that if the party insiders want to hurt Sanders, don't have Hillary Clinton slam him; have Rahm Emanuel and Steny Hoyer endorse him and praise him as someone who talks a fiery game but can be counted on to make compromises at crunch time. In the same way that Trump gets strength from traditional media outlets criticizing him, Sanders supporters are not people you win over by touting the endorsement of the New York Times. You're talking about a campaign, and a following, that legitimately thinks it's awesome that Barack Obama is (reportedly) uncomfortable with a Sanders candidacy.
In short, I'd recommend the second option which would be the most challenging but also the most likely to succeed in blocking a Sanders campaign that started racking up wins. The only good play is to quickly and decisively pick one alternative and unite everyone whose vote was divided among the majority "Not Sanders" enthusiasts supporting other candidates. It would take unprecedented politicking behind the scenes, and a party-first attitude from some of the candidates, to make this work. That said, grander and more challenging political bargains have been struck throughout American history, and if the Democratic mainstream really does perceive Sanders as an existential threat then insert cliches about desperate times here.
Lots of politics junkies claim to have a fondness for the old school horse-trading politics of the pre-1968 era. While primary voters will always be a wild card, the powers that be within the party have a chance to prevail upon the field of candidates that a crisis is at hand.
Personally I doubt the people in present party leadership positions (official and unofficial) have the decisiveness and forcefulness necessary to pull this off. But it is what I'd advise them to do, and were it to work it would solve the problem with far more certainty than the other options.