WATCH: Top 5 moments from the 2nd Democratic debate

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In the wake of the Paris terrorist attacks, Democratic presidential hopefuls Hillary Clinton, Vermont Sen. Bernie Sanders and former Maryland Gov. Martin O'Malley faced off in a national security-focused debate at Drake University in Des Moines.
Here are some of the best lines from the debate, hosted by CBS News:

1. O'Malley tries his hand at name-calling. When addressing the topic of compromising with Republicans on immigration, O'Malley didn't hold back, bashing Republican rival Donald Trump.

"If more border security and ... and more and more deportations were going to bring our Republican brothers and sisters to the table, it would have happened long ago," he said. "The fact of the matter is – and let's say it in our debate, because you'll never hear this from that immigration-bashing carnival barker, Donald Trump – the truth of the matter is net immigration from Mexico last year was zero. Fact check me. Go ahead. Check it out."

>> Click here to watch the clip

2. Sanders slams chatter about Clinton's emails ... again. At last month's first Democratic debate, Sanders made headlines and thrilled social-media users by saying of the Clinton email scandal, "The American people are sick and tired of hearing about your damn emails."

Well, apparently, that's still true.
"I was sick and tired of Hillary Clinton's email," he said when the topic came up again at Saturday's debate. "I am still sick and tired of Hillary Clinton's emails."
"What I would like for the media now is for us to be talking about why the middle class is disappearing, why we have more people in jail than any other country, why we have massive levels of income and wealth inequality, and we're the only major country on Earth without paid family and medical leave," he added.
"We've gotten off the Hillary's emails, good. Let's go to the major issues facing America."
Clinton responded, "I agree completely."

>> Watch the video here (start at 2:34 timestamp)

3. Sanders ties terrorism to climate change. The senator doubled down on earlier statements that climate change is the greatest threat to national security – and went even further by saying the two threats are connected.

"Climate change is directly related to the growth of terrorism," he said Saturday. "And if we do not get our act together and listen to what the scientists say, you're going to see countries all over the world – this is what the CIA says – they're going to be struggling over limited amounts of water, limited amounts of land to grow their crops, and you're going to see all kinds of international conflict."

>> Click here to see the clip

4. Clinton: Sanders is trying to "impugn my integrity." The debate heated up when Sanders slammed Clinton for accepting donations from Wall Street. 

"I have never heard a candidate never, who has received huge amounts of money from oil, from coal, from Wall Street, from the military industrial complex, not one candidate say, 'Oh, these campaign contributions will not influence me. I'm going to be independent,'" Sanders said. "Well, why do they make millions of dollars of campaign contributions? They expect to get something. Everybody knows that."
He added, "Once again, I am running a campaign differently than any other candidate. We are relying on small campaign donors, 750,000 of them, 30 bucks a piece. That's who I'm indebted to."
Clinton fired back, "He has basically used his answer to impugn my integrity. Let's be frank here."
She then brought up the Sept. 11, 2001, terrorist attacks to explain her position. 
"So, I represented New York, and I represented New York on 9/11 when we were attacked," she said. "Where were we attacked? We were attacked in downtown Manhattan where Wall Street is. I did spend a whole lot of time and effort helping them rebuild. That was good for New York. It was good for the economy, and it was a way to rebuke the terrorists who had attacked our country."
At least one Twitter user wasn't buying Clinton's answer, saying, "I've never seen a candidate invoke 9/11 to justify millions of Wall Street donations until now." 
When asked to address the tweet, Clinton responded, "Well, I'm sorry that whoever tweeted that had that impression because I worked closely with New Yorkers after 9/11 for my entire first term to rebuild. So, yes, I did know people. I've had a lot of folks give me donations from all kinds of backgrounds say, 'I don't agree with you on everything, but I like what you do. I like how you stand up. I'm going to support you,' and I think that is absolutely appropriate."

>> Watch the video here

5. Sanders gets feisty about gun control. O'Malley touted Maryland's gun-control measures, to the chagrin of the Vermont senator.

"In my own state, after the children in that Connecticut classroom were gunned down, we passed comprehensive gun-safety legislation with background checks, ban on assault weapons," he said.
He added, "There's a big difference between leading by polls and leading with principle. We got it done in my state by leading with principal, and that's what we need to do as a party for comprehensive gun safety."
But Sanders shut him down.
"I think it's fair to say that Baltimore is not now one of the safest cities in America," he quipped.

>> Click here to see the exchange (start at the 11:51 timestamp)