I've read up on this.

The answer is simple. Obama wants ANY democrat to win. If he backs one now...the others will feel neglected and sad...and if Obama's early -backed candidate bombs and fails to get the nod in July...then his endorsement of an "also-ran" will be laughed at.

He has to wait.

You can bet that once we have a clear nominee, Barack will come out with both guns blazing.

@Glennlab, below, says pretty much the same thing.

Obama, unlike the orange menace, is the consumate politician and human being, for that matter. He will wait til whomever wins to endorse. Maybe, just maybe, in a brokered convention he would speak out.

I think the idea that Sanders isn't very electable is turning out to be a myth. He is the most popular Senator in the nation. He made history by being the first person from either party to win all three of the first states in the primary. He is way ahead in all of the polls. He has an army of enthusiastic supporters willing to do whatever it takes to win. He is leading with people of color. People trust him the most on the most important issue of this election, healthcare. He has energized the youth vote. He does very well with working class voters of the rust belt which is the exact demographic which caused Hillary to lose. He is outside of the establishment and has been an independent for most of his career and independents like that so he does well with them. He can take voters away from Trump because he too is a populist who appeals to working class voters who feel left behind. He is shattering individual donor records and people love the fact that he is funded by the people rather than by the special interests. He is filling stadiums when he gives speeches.

Trump himself was caught on tape talking about how running against Bernie's popular policies will be very difficult. To beat Trump and take back the Senate, we need high voter turn out. A moderate like Joe does not inspire the kind of enthusiasm to get it done.

We ran a moderate last time and lost. In fact, every moderate we have run this century has lost. Gore- lost, Kerry- lost, Obama- won but ran on a progressive hope & change message, Clinton- lost. Why repeat that same failed strategy when we have a very unique candidate who has motivated millions of Americans to become politically active? Bernie will get the vote blue no matter who folks because they just want to vote against Trump and don't care who it is. He has the Hispanic vote on lock down. He is extremely popular with the youth, so much so that they are turning out for him on the same level and perhaps more so than they did for Obama. He is even doing well with moderates and conservatives because people of all ideologies recognize and respect his, authenticity, consistency, and integrity.

If we want to beat Trump, Sanders is the candidate most likely to be able to do it.