I'm not too certain that will be the case. It's still only February, and there's right now a bit of a shockwave going Sanders supporters that he might not win this. People are disappointed, claiming they'll abstain, vote third-party, or even try to rile-up Clinton supporter by making dubious claims they'll vote for Trump. I feel a lot of it is just the venting of frustrations. We saw this eight years ago when allegedly Clinton supporters were going to either abstain or vote McCain in droves, costing Obama the election. Didn't happen.
It's still only February. After having time to process this, and more importantly watch a few months more of the campaign trail where a Trump, Cruz, or Rubio presidency might start looking like a twinge of a reality, and watch the number of those abstainers start to flock back to the Democrats. Let's observe this phenomenon after the conventions. Let's observe this following a debate or two of Hillary versus [GOP Candidate].
Besides, one could say divisions are high among GOP too.
The thing you have to take into account is, this is not a simple ideological disagreement within a party about policy for many in the Sanders camp. That is where Hillary Clinton would be blindsided in a general, and what i think a lot of people who are not Sanders supporters tend to miss that when they take voting blocs into account.
Obama fanboys and Hillary fanboys, and Obama and Hillary themselves were far closer than anything on the table today, and they were in the same party regardless of how ugly their rhetoric got
For most Bernie Sanders supporters specifically, this election is not about 'free stuff' or 'sky high promises' or specific nuances between the candidates in policy at all. Most of his support comes from people who are serious about wanting a clean break from choosing corrupt candidates who are not genuine about their intentions for the country, we can see that also in another way with Trump, although they are completely on the opposite side.
In Hillary, they don't 'see' a candidate who is a moderate center right candidate that pushes for legislation that was GOP legislation just a few years back, and is hawkish on foreign policy as we've grown used to from the new Democrats.
If that was the only problem, i think this would turn out to be a repeat of Obama 2012, where he still won handily.
For many people supporting Sanders, choosing Hillary is tantamount to supporting a broken system with revolving door influences with many special interests and severe conflicts of interest in almost every area of government between the political establishment and the corporate establishment, rigged against the ordinary lower class citizen.
And because of that, there WILL be a large amount of people who either not vote on principle, vote for Trump out of spite, or become disillusioned with the political process again and just drop out of politic circles. Those new voters Bernie brought into the political process because of his honesty aren't staying around and voting for her, unfortunately they were the first time younger voters who were really enthused about him, consistently giving him over 70% of the younger vote, and aren't enthused about anything regarding Hillary.