You can criticize him for proposing "racial profiling" to thwart terrorism. You can point out his back-and-forth positions on gun control and safety. You can note how poorly he's polling among Latinos and women. But if you really want to get Donald Trump's goat, call him small, weak or insecure.

After watching Trump astound the establishment political class by winning more primary votes than any Republican in history, foes have homed in on what really makes Trump look away from the newly present teleprompter and spout off, either on Twitter or in unscripted remarks. And that's anything that questions the very bigness of Trump, be it his wallet, his deal-making skills, his ability to win and even his hands (and whatever else that might suggest).

"He does have some buttons that really make him say inappropriate things, when they are pushed," says psychologist A.J. Marsden, a professor at Beacon College in Leesburg, Florida. "To him, being a loser is much worse than being a bigot. In business, you can get called a bigot and a sexist, and he kind of lets that roll off his chest. He's more concerned with winning and making money," Marsden adds.

The idea is to get Trump to explode, to make a comment (such as his calling Sen. Elizabeth Warren of Massachusetts "Pocahontas") that will insult or concern a swath of voters, peeling off potential support for the famously bombastic real estate magnate. Trump has recently taken to reading prepared remarks from a teleprompter, but when he's provoked, he goes off the GOP talking points and takes to social media. Seeing Trump look disturbingly un-presidential, his foes believe, will deny him the role.

It's that theory that appears to be leading a U.S. senator and former Harvard law professor to attack Trump not on his policy proposals, but on his very Super Trump-iness. "If you think recycling Scott Brown's hate-filled attacks on my family is going to shut me up, @realDonaldTrump, think again buddy. Weak," Warren tweeted, using one of Trump's own favorite insults to counter his charge that she called herself part Native American to advance at Harvard. Even when Warren does throw in a few campaign issues, she finishes with a signature Trump jab: "@realDonaldTrump can't talk about Wall St, college costs or min wage, so he spent all day belching insults. Pathetic. Really pathetic," Warren tweeted in May.

On the non-virtual stage, too, Warren has gone after Trump as the human brand.

"Every day we learn more about him, and every day it becomes clearer that he is just a small, insecure money-grubber who doesn't care about anyone or anything that doesn't have the Trump name splashed all over it," Warren told New Hampshire Democrats last weekend.

"Calling Trump a misogynist doesn't nail him," says Joel Weinberger, a psychology professor at Adelphi University. "What Elizabeth Warren is looking at it – he's a billionaire with a beautiful wife, and he's going to remind you every two minutes? He's insecure. She gets it," Weinberger adds.

Then there's the matter of the hands. That insult-swapping started some 30 years ago, when then-Spy magazine editor Graydon Carter called Trump a "short-fingered vulgarian." Trump, Carter said, was so defensive about it that he has sent pictures of himself to Carter (one as recently as last year), circling his hands with a gold Sharpie and insisting his fingers were not abnormally small.

GOP primary opponent Sen. Marco Rubio of Florida commented on Trump's hand size, suggesting that meant Trump also had small genitalia. That, too, set off Trump, who brought the matter up during a Republican debate.

"Look at those hands, are they small hands?" Trump said, lifting them for the audience to examine. "And, he referred to my hands – 'if they're small, something else must be small.' I guarantee you there's no problem. I guarantee."

An anti-Trump political action committee calling itself Americans Against Insecure Billionaires with Tiny Hands has resurrected the issue, running a TV ad that starts out like a serious issue-oriented spot, then asks, "if the White House phone rings at 3 a.m., will his little hands even pick up the receiver? How can he create jobs when his hands are too small to shake on the deal?'

Katie Nguyen, the group's communications director, says the point of the ad is twofold. "On the one hand, we are genuinely concerned about the size of Donald Trump's hands and his capabilities, or incapabilities, to be president of the United States. It's not body-shaming. I have tiny hands myself," Nguyen deadpans. Second, she says, "Donald Trump's response to his hand size has been pretty alarming. He's gotten very defensive," saying something troubling about his temperament.

And that, Weinberger says, is why Warren's tweets have been so well-targeted. "It's not his business acumen or issues. It's the size of his package that it all comes down to," Weinberger says. Whether it's his bank account, model wife or hands, "he's saying, I've got a bigger one – look at me.' She hits him there," Weinberger says.

Trump, too, has tapped into human psychology to get to where he is, experts say. American voters have been described as angry, but they're really disgusted, Weinberger says. Trump's use of the word "disgusting" to describe people or thigs that bother him (including a woman who wanted to pump breast milk during a break from a court proceeding, and Hillary Clinton, using the bathroom during a break at a debate) go to that, he said.

Trump's labeling of his opponents – "Little Marco" (Rubio), "Lyin' Ted (Cruz), "Low-Energy" Jeb Bush and "Crooked Hillary" – also highlight what Marsden says is Trump's cunning ability to "control the conversation" and brand his foes.

As for Warren, Trump has responded to the senator with the aforementioned "Pocahontas" label and by calling her "goofy" – not helpful to a candidate establishment Republicans are trying to make more presidential-looking. And the Massachusetts lawmaker responds in kind.