Later in the two-hour debate, Harris hopped into a conversation on race and directly targeted former Vice President Joe Biden over his past work with segregationist senators.

“As the only black person on this stage, I would like to speak on the issue of race,” Harris began before MSNBC moderator Rachel Maddow said she could have 30-seconds to respond.

Harris then spoke for an uninterrupted 107 seconds.

“Growing up, my sister and I had to deal with the neighbor who told us her parents couldn’t play with us because she — because we were black. And I will say also that — that, in this campaign, we have also heard — and I’m going to now direct this at Vice President Biden, I do not believe you are a racist, and I agree with you when you commit yourself to the importance of finding common ground,” Harris said directly to Biden.

“It was hurtful to hear you talk about the reputations of two United States senators who built their reputations and career on the segregation of race in this country. And it was not only that, but you also worked with them to oppose busing,” she said about the Department of Education integration policy of busing to integrate schools during the 1970s.

“And, you know, there was a little girl in California who was part of the second class to integrate her public schools, and she was bused to school every day. And that little girl was me,” Harris said.

Biden then tried to fire back at Harris, saying that it was a "mischaracterization of my position across the board."

"I did not oppose busing in America," Biden replied. "What I opposed is busing ordered by the Department of Education, that's what I opposed."

After a brief back-and-forth about civil rights, Biden then took a shot at Kamala Harris’ work as a prosecutor.

“If we want to have this campaign litigated on who supports civil rights and whether I did or not, I’m happy to do that,” Biden began.

“I was a public defender. I didn’t become a prosecutor. I came out and I left a good law firm to become a public defender, when, in fact — when, in fact, my city was in flames because of the assassination of Dr. King, number one. Number two, as the U.S. — excuse me, as the vice president of the United States, I worked with a man who, in fact, we worked very hard to see to it we dealt with these issues in a major, major way,” he added.

The exchange between Harris and Biden quickly became a trending topic on Twitter, where there were both tweets in support of Harris as well as tweets calling out her actions as a prosecutor.

Another notable moment in the debates occurred when South Bend, Indiana, Mayor Pete Buttigieg said he wishes he could do more to diversify the police force and mitigate racial tensions that erupted when a white police officer shot and killed Eric Logan, a Black man in Indiana.

"We are hurting. I could walk you through all of the things we have done as a community," Buttigieg said. "All of the steps we took, from bias training to de-escalation, but it didn't save the life of Eric Logan. When I look into his mother's eyes, I have to face the fact that nothing that I say will bring him back."

"We've taken so many steps towards police accountability," he said, adding that the police labor union "denounced me for too much accountability, we're obviously not there yet, and I accept responsibility for that because I’m in charge."

However, California Rep. Eric Swalwell, another candidate on stage, challenged Buttigieg by saying he should fire the police chief if the officer who killed Logan did not have his body camera during the incident.

"So under Indiana law, this will be investigated, and there will be accountability for the officer involved," Buttigieg began.

"But you're the mayor, you should fire the chief, if that's the policy and someone died," Swalwell rebutted.

During the debates, Swalwell also took a jab at Biden's age.

"I was 6 years old when a presidential candidate came to the California Democratic Convention and said it's time to pass the torch to a new generation of Americans," Swalwell said. "That candidate was then-Sen. Joe Biden. Joe Biden was right when he said it was time to pass the torch to a new generation of Americans 32 years ago. He is still right today."

Biden responded by saying, "I'm still holding on to that torch," and then quickly began speaking about his education plans.

In addition to the exchanges between Sanders, Harris, Buttigieg and Swalwell, Bernie Sanders used his time during the debate to advocate for a Medicare for All plan that would inevitably raise taxes on the middle class.  

"We have a new vision for America," Sanders said. "At a time when we have three people in this country owning more wealth than the bottom half of America while 500,000 people are sleeping on the streets today. We think it is time for change."

The final candidate who appeared to win Twitter for the evening was author, lecturer and activist Marianne Williamson. While she may not have had many opportunities to speak, Williamson certainly made sure that each other's talking points would become viral moments on their own.

By the end of the night, Williamson’s Twitter mentions were all over the place.