This needs words...
I don't know who created this dishonest chart, because it is not sourced. But it is a deliberately dishonest manipulation of a legitimate chart created by Nate Silver in 2012. The key to the dishonesty is in the timing, as described at the top of the chart:
Last-Stage (September-November) Favorability Ratings NYT/CBS News Polls since 1980
And then current favorability ratings of Hillary Clinton and Bernie Sanders are edited in, supposedly showing that Clinton's numbers make her unlikely to win a runoff, while Sanders is likely to win. The problem is that the legitimate chart is from the months immediately before runoff elections, and we are now some ten months away from next year's. Polls can change in those ten months. In fact, history shows exactly how much polls can change. Candidates typically get a large favorability spike from their conventions, which is when the runoff really begins. And the deliberate dishonesty of whoever created the dishonest chart is revealed in the fact that the same article by Silver includes another legitimate chart, which better represents the current time frame, and provides a better view on the relevance of current favorability polls. And it doesn't promote the narrative that the creator of the dishonest chart wants to promote.
Notice that this second chart shows the months January through June, rather than September through November. Notice that during the relevant time frame, Bill Clinton in 1992, Al Gore in 2000, the Lesser Bush in 2004, President Obama in 2012, and the elder Bush in 1988 all had under water favorabilities in the January to June window, and all went on to win the general election popular vote, with all but Gore becoming president. Note that of those with above water favorabilities, Michael Dukakis in 1988, President Ford in 1976, John McCain in 2008, Walter Mondale in 1984 and John Kerry in 2004 all went on to lose. In other words, favorability ratings in the first half of an election year tell us exactly nothing about the "electability" of the candidates. But that is not what the creator of the dishonest chart wants you to believe.
Silver was analyzing the forecasting value of favorabilities and economic factors, specifically regarding Mitt Romney's early negative favorabilities at the same time a tepid economy seemed dangerous to President Obama's re-election prospects. He wrote:
Still, this leaves much open to interpretation: are favorability ratings an important causal factor in the vote or, instead, more just a correlate of them? If, for instance, elections are dictated by economic fundamentals, it is likely that voters will come to view the incumbent favorably when it is easier to find jobs. It might also be the case that they will come to view the challenging candidate less favorably under these circumstances since he will have a tougher argument to make and may have to resort more to negative campaigning and personal attacks.
And:
There have been clear reversals in favorability ratings in the recent past once the general election campaign got under way, such as in 1988 and 1992. At least one recent candidate (Mr. Clinton in 1992) won his election with similarly mediocre early favorability ratings. With that said, it would be foolish to suggest that this makes no difference at all.
And:
It has not been uncommon in the past for a candidate’s numbers to decline while he is actively engaged in a primary, but for him to go through a honeymoon period once he begins to wrap up the nomination.
His conclusion?
For the time being, I’d consider them a negative but fairly minor factor when we evaluate his chances at victory.
Of course, many factors played out, including Romney's inept campaign, but by election day Romney's favorability ratings were slightly lower than President Obama's but with a higher spread. And the economy still wasn’t booming. And we know who won.
One of the more revealing instances of early favorability ratings having no predictive value is from Bill Clinton, in 1992, because we have a steady flow of data, from Gallup.
Note that during the early part of the year, Clinton’s approval rating often was under water, and was 8 points under (41-49) heading into his convention, which was held July 13-16. He came out of his convention up 37 points (62-25) and never looked back.
The bottom line is that favorability ratings at this point tell us nothing at all about next November's runoff, and whoever inserted the current favorabilities of Hillary Clinton and Bernie Sanders into a chart representing favorabilities from the few months directly before past runoff elections was being deliberately manipulative and dishonest.