Opponents of Trump's tried to say the image looked like a Nazi salute. (Rival Ted Cruz tried to take advantage of the attention-grabbing gesture, turning it on Trump this way at Thursday's debate: "This is a job interview. We are here pledging our support to you, not the other way around.")
So, cue another round of "Trump is Hitler" think pieces and tweets and cable news conversations.
So far, celebrities, like conservative talk-show host Glenn Beck and comedian Louis C.K., have compared Trump to Hitler, and even world leaders, past and present, like Mexican President Enrique Peña Nieto and former Mexican president Vicente Fox have done it, too.
"I don't want to offend anybody, but I can tell you that it's been amazingly received," Trump said of the hand-raised pledge. CNN reported that Trump said, "I don't know about the Hitler comparison. I hadn't heard that, but it's a terrible comparison. I'm not happy about that certainly." He continued, "Well, I think it's [the comparisons] ridiculous, I mean we're having such a great time."
Remember Godwin's Law
It's important to note that Trump is not the first man to be compared to the former German leader, and he will not be the last.
In fact, there's an entire Internet law all about Hitler/Nazi/Holocaust comparisons. It's called Godwin's Law of Nazi Analogies. It's pretty simple: "As an online discussion grows longer, the probability of a comparison involving Nazis or Hitler approaches one."
Mike Godwin created that theory in 1990. So, as fresh as this outrage may seem, it is actually just a new iteration of an online trope. And it's more than Hitler comparisons. Remember when people were comparing Obamacare to slavery?
Kerric Harvey, author of the Encyclopedia of Social Media and Politics, says there's a problem with these types of comparisons.
"It oversimplifies both sides of the comparison," she told NPR. "When a conservative says, 'Obamacare is slavery,' they're really oversimplifying both Obamacare and slavery in ways that are not helpful.... And they actually sort of cover up the kinds of nuanced, important points in each side."
Harvey also says these types of comparisons rarely accomplish the attempted goal: getting people to change their minds on an issue. "If you call Trump Hitler to a Trump supporter, it's actually the case that they're just going to dig in their heels and support him even more," Harvey said, "because they will find that comparison so outrageous.
"The metaphor almost works against itself if what you're really trying to do is explain what you mean. If what you're trying to do is arouse knee-jerk reaction, at an extreme level, then that kind of comparison does a lot of work for you. But does it add a lot of good to ongoing public discourse? I don't think so."
But Godwin says there are times the comparison is OK
Godwin told NPR that comparing something or someone to Hitler isn't always bad.
"Well, I never say that the comparisons shut down the argument," he said. "Other people have said that maybe it does. And it's certainly true that people interpret Godwin's Law to say that once you get to the Hitler comparison, the meaningful discussion is over. [But] I don't think that's true.
"What you want to see happen is people, if they're going to make a historical reference like that, they have to actually know what they're talking about. You have to ask people, 'How can you say that, or what specifically are you basing this on,' and force them to get to the facts."
But, in an age of social media, that might be increasingly difficult.
'Novel-length issues by the first line of a haiku'
Kerric Harvey, says there are two reasons this is the case. First, brevity.
"Almost all of our social media are ways of reacting to complicated long-term enduring social issues and political tangles," Harvey said, "in very short times. We have 140 characters [on Twitter]. We have one picture [on Instagram]... We've got, what is it, [in a Vine loop] seven seconds? We try to deal with novel-length issues by the first line of a haiku."
She also says there's a pressure to make all those responses, all those Hitler memes as quickly as you can.
"The knee-jerk aspect, the immediacy that our social media don't just permit us, but almost demand of us," she said. "And that is not built into the technology. That is sociological. We have decided that because we can respond right away, we're going to respond right away."
When asked if this might change anytime soon, Harvey said no.
"It's not impossible, but it feels impossible," she said. "It's like trying to slam on the brakes on the Titanic, so you don't hit that iceberg."