On ABC's This Week, Bernie Sanders spoke in clear terms, stating that a vote for someone other than Hillary Clinton is a vote for Donald Trump.
Here is Bernie Sanders speaking as directly as he ever has on the choice facing voters in November:
— ABC News Politics (@ABCPolitics) October 2, 2016
The evidence is overwhelming that the next president of the United States is going to be Hillary Clinton or Donald Trump. I think if you're voting for somebody else, in a sense, and not supporting Clinton because she doesn't live up to all of your specifications or all of your ideas, I think in a sense it is a vote for Trump.
This echoes similar comments from President Obama:
If there’s one message I want to deliver to everybody: If you don’t vote, that’s a vote for Trump. If you don’t vote, that’s a vote for Trump. If you vote for a third-party candidate who’s got no chance to win, that’s a vote for Trump.
Sanders also emphasized that Gary Johnson's record is too conservative for his supporters, specifically on the environment:
I think also, take a look at Gary Johnson's record on the environment, on the economy. It is a very conservative approach, something that I think most of my supporters do not support.
Paul Krugman concurs.
What really struck me, however, was what [Johnson's] platform says about the environment. It opposes any kind of regulation; instead, it argues that we can rely on the courts. Is a giant corporation poisoning the air you breathe or the water you drink? Just sue: “Where damages can be proven and quantified in a court of law, restitution to the injured parties must be required.” Ordinary citizens against teams of high-priced corporate lawyers — what could go wrong?
The fact is this: The true protest vote in 2016 is a vote FOR Hillary Clinton.