First of all, it's only October. 2019. To say any one of the Democratic candidates is the likely nominee is premature.
Paraphrased: "Don't make any predictions/assumptions of future outcomes where there is a possibility of doubt"
Second, Donald Trump, if he had any chance at all of being reelected, just lost the 2020 election yesterday as Turkish forces were attacking the Kurds in northern Syria. Of all the stupid things Trump has done since taking office, pulling our troops in Syria and allowing what will become a bloodbath (not to mention in effect setting 12,000 or more ISIS murderers on the loose) is by far his most heartless and idiotic move yet.
Paraphrased: "Here's a prediction/assumption of a future outcome in which I'm assuming everyone feels the way I do"