Donald Trump is looking at some polls, though. Not all of them. If he were looking at all the live-caller polls, he would see something that looked less like "Donald Trump will beat Hillary Clinton" than "Hillary Clinton will beat Donald Trump."
Real Clear Politics has Clinton as a six-point favorite based on its poll average, which includes the new Post/ABC poll putting Clinton up by five points. If the election were held tomorrow, the saying goes, Trump would very likely lose.
Maybe! Who knows! The election isn't tomorrow and there are so many factors that come into play. Even that graph above doesn't tell us a whole lot. It says that in some polls, Clinton is preferred over Trump -- but in a few she isn't.
Compare that to a similar set of polls looking at Clinton vs. Marco Rubio.
Here, Rubio generally does better, but not always.
There's another layer to this that's important to note: This doesn't tell us anything about who will win next year because we still don't know who's going to win the primaries.
If you showed me those graphs and said, Philip, will our next president be a Democrat or a Republican?, I would say, "How should I know?" I would then look at you with a skeptical look and say, "This doesn't really tell us anything, you realize."
Let's use another example: Polling from the middle of December 2007.
At that point, Gallup did a survey to pit possible Democratic candidates against possible Republican ones. The Democrats did better, across the board, but by between 5 and 18 points, depending on the match-up. It was not very predictive.
But also notice something: They didn't ask about John McCain! McCain at that point was polling fourth in the Real Clear Politics average, behind Giuliani, Huckabee and Romney. So why ask about that nobody, right?
A month later, McCain was in the lead, and Gallup asked about him. This was as he was starting to lock up the race and while the Democrats were still duking it out. McCain beat both Democrats, in part because he had more than 75 percent support from Republicans and Obama and Clinton got 53 and 71 percent of the Democratic vote, respectively. But of course, Obama won easily, thanks to Democrats coalescing around him after the primaries (and, of course, the obliteration of the economy).
So many things come into play in a general election. It is a very good idea to wait and see who the candidates are and how the campaign (and external factors) progress before swearing up and down that you know who will win.
Unless swearing up and down on limited evidence is your thing. In which case, tweet away.