Trump is spouting racist rhetoric popular a century ago, like "racehorse theory" — see Trump Praises Good Genes for Haaretz's comparisons to the Nazi era.
Trolling Techniques: (1) Outrage —> (2) Anger —> (3) Circle the Wagons cycle
How do politicians use racism to get people who voted for Obama to vote for them? Today people who earnestly see themselves as not racist, who take offense at the accusation, keep supporting a politician who built his base with the Birther movement. What's the political trick?
The strategy has three steps, and has been dominating politics since at least the "deplorables" moment.
(Step 1) The provocateur politician says something offensive, racist, sexist or xenophobic — ideally something that people who are not politically active and not engaged in fighting racism won't immediately be appalled by, even if they wouldn't like it if they paid more attention.
(Step 2) Progressives get angry and light a huge media fire. To anyone not paying attention, not following the news closely, progressives will make a lot more noise than the original outrage.
(Step 3) Convince people who weren't as appalled as progressives are, that the progressive anger is aimed at them, and welcome their votes.
You see this strategy over and over: the way the "deplorables" quote took examples of racism that most Americans do not want to be associated with, and Fox and Trump made all Trump voters feel like it was aimed at them, even though the long form of the quote very explicitly was not. "All Lives Matter" is such a nice phrase when it's seen in a vacuum, so less active people say it, and then wonder why anger is aimed at them.
So far, the left has been pretty ineffective at countering this, which I believe is one reason that self-interested politicians continue to say things that progressives will find racist and white people who'd prefer to ignore politics (and see "racist" as an insult, do not wish to be nor be seen as racist) do not.
Dog whistle politics has changed. In the past, a "dog whistle" was when a politician said something that his supporters would know was racist, but centrists wouldn't hear it. Only supporters, only the target demographic that the speaker intended to mobilize, heard the dog whistle.
I watched good folks start to tear into each other when this window decal made its way to a political facebook group.
I'm guessing: this image is seen as wildly racist to some, and a silly joke about lousy ex-boyfriends to others.
And there is a big danger: when people don't think they're sharing something racist, and then get accused of it, they tend to feel pushed out, not called in. This is creating, over and over, a feeling of rejection among a large part of the country.
This provocation is done very much on purpose: