1) Donald Trump is going to be the Republican nominee;
2) In the choice between Hillary and Bernie, the top priority should be who is most likely to defeat Trump in November.
I've encountered many who believe that the priority on winning makes it crystal clear that the Democratic nominee should be Hillary. They may be right. But it is far from obvious, to me, that they are. It's a question on which it's worth keeping an open mind.
Here are some thoughts intended to invite such people to give the question further consideration:
This means that Hillary's sweep of the reliably-Republican Southern states (as well as Bernie's victory in Oklahoma) is meaningless for November. What matters for victory is which candidate can hold onto all the blue states and carry enough of the battleground states to get to 271 electoral votes. In such states, thus far, Bernie and Hillary are showing relatively equal strength (close finishes in Iowa, Nevada and Michigan, for example, with a strong victory for Bernie in New Hampshire and for Hillary in Virginia).
While the two have been generally even, however, they are not drawing their strengths from identical demographic groups. As we all know, Hillary has a huge advantage among African-Americans. Among Bernie's big strengths is support from "independents" and from the young.
While it is true that Bernie is not, and likely will not become, the favorite candidate for African-Americans, it seems unlikely they will stay home in November. Bernie will not be the positive draw for them that Barack Obama was, of course, or that Hillary might be. But there should be compensation for the loss of that source of enthusiasm from the negative motivational force of Donald Trump as the alternative. Whoever is the Democratic nominee, I would guess that African-Americans (and Hispanics) will be motivated to go to the polls to prevent our having a President Donald Trump.
The independents, on the other hand, may be up for grabs. Chris Hayes (on MSNBC) has discovered a slice of the electorate that is torn between Bernie Sanders and Donald Trump. These voters are pretty certainly not going to be Clinton voters.
Another difference between the two has to do with the age of voters: Hillary is strong with older voters, Bernie has an overwhelming advantage with voters under 30. Now, which of those two groups can be counted on to go to the polls? Yes, the older voters who vote for Hillary are far more reliable participants in the electoral process--and they are not going to vote for Trump. So the Democrats can probably count on those votes. But, if Hillary is the nominee, the younger voters who prefer Bernie will not likely vote for Trump over Hillary, but history suggests a sizable fraction of them will not vote at all.
It is well established that current polls of the American electorate as a whole show Bernie performing better against Trump (and against any other potential Republican nominee) than Hillary. Supporters of Hillary argue, quite rightly, that what such polls show now is of dubious relevance to how things will develop once the general election process begins after the convention.
In particular, it is argued -- also rightly -- that Bernie has not yet been the target of the kind of well-funded smear campaign that the Republicans are sure to unleash upon him if he were to become the nominee. By contrast, Hillary has already been the subject of vicious right-wing attacks for years, even decades. And so, this plausible argument concludes, Hillary is relatively immune to any more such attacks. Her poll numbers already have the impact of that kind of negativity baked into the cake, while the inevitable attacks on nominee Bernie will take a toll on his numbers.
I must say, I share that concern.
In particular, having watched since the 1950s how the Republicans have used the term "socialist" as a bludgeon against anyone who argues for public goods or, heaven forfend, for any kind of redistribution of wealth, I am fearful about how effectively the Republicans might wield that word against Senator Sanders. Moreover, I've not been all that impressed with the ways Bernie has dealt so far with this potential problem.