How Hillary Clinton Got Under Donald Trump's Skin During the Debate There were a couple of touchy subjects.

 -- One of the big questions Monday night was how Hillary Clinton and Donald Trump would try to goad each other into losing their cool during their first face-off.

Trump was known for going off on his foes during the primary debates, so the expectation was that Monday night would be no different. And there were several times he seemed to come close.

Aaron Kall, the director of debate at the University of Michigan, said he thought Clinton’s attack on Trump's tax returns "was the most effective way" she got under his skin.

Clinton suggested a few reasons Trump might be refusing to release his returns, and after she charged that he didn't pay federal income taxes in some years, he couldn't stop himself from boasting, "That makes me smart."

"He's very protective of his image and of his bravado, and so when you attack someone on his perceived strengths — his business acumen — then he's going to get very defensive when that's on the table," Kall told ABC News.

Kall also pointed to Clinton's invoking Michelle Obama's convention speech as another subtle jab, not only because the first lady is a popular figure on both sides of the aisle but also because of the controversy that surrounded Melania Trump's convention speech, which echoed one of Obama's earlier speeches.

Kall called the reference "politically smart" and said it could have helped "in getting under his skin."

James Campbell, a professor of political science at the University at Buffalo in New York who has written a book about political polarization, said it was clear that Trump was bothered by some of the topics that came up.

"I'm not sure whether Trump went off the rails because of Clinton, the moderator or just his personality. The contrast to me of how the candidates dealt with personally related problems was stark," Campbell told ABC News.

"Clinton dismissed the most serious charges regarding her private email servers being a possible breach of national security by simply apologizing for it. One 'Oops,' and then the issue is gone — at least for this debate. Smart move if you can get away with it, which she did," he said.

“Trump, on the other hand, is so aggressive and unapologetic that he relitigates issues — birtherism, Iraq War opposition — that he has no possibility of winning on. As a result, he is on the defensive longer and highlights his problems.”