October Democratic debate: 5 memorable moments from Tuesday's matchup

1. Warren fends off more attacks following her rise in the polls.

After passing Biden as the front-runner in some national polls, Warren drew more criticism from other candidates Tuesday night – most notably for her support of "Medicare for All."

When asked whether the health care plan would raise taxes, Warren didn't directly answer but stressed that overall "costs will go down for hardworking, middle-class families." Her response came just moments after Sanders conceded that taxes would be higher under the proposal, but that the increase will be "substantially less" than the costs of "premiums and out-of-pocket expansions" that people currently face.

Klobuchar, a "Medicare for All" critic, took aim at Warren's remarks.

"At least Bernie's being honest here and saying how he's going to pay for this and that taxes are going to go up," she said. "And I'm sorry, Elizabeth, but you have not said that, and I think we owe it to the American people to tell them where we're going to send the invoice."

Klobuchar later added: "I appreciate Elizabeth’s work, but, again, the difference between a plan and a pipe dream is something that you can actually get done."

>> Watch the moment here

2. Sanders discusses his health. 

After Burnett asked Sanders, who suffered a heart attack earlier this month, how he can reassure voters that he's "up to the stress of the presidency," the 78-year-old senator promised that he will "be mounting a vigorous campaign all over this country."

"That is how I think I can reassure the American people," he said.

He then thanked backers and fellow candidates for their support during his hospitalization.

"Let me take this moment if I might to thank so many people from all over this country, including many of my colleagues up here, for their love, for their prayers, for their well-wishes," Sanders said. "And I just want to thank you from the bottom of my heart, and I'm so happy to be back here with you this evening."

>> Watch the moment here

Sanders' remarks followed a humorous exchange with Booker after moderator Erin Burnett asked Sanders about his health.

"I'm healthy. I'm feeling great, but I would like to respond to that question," Sanders said, referring to a previous question about pharmaceutical companies.

Booker interjected: "And Sen. Sanders is in favor of medical marijuana. I want to make sure that's clear, as well."

"I do," Sanders said, adding, "I'm not on it tonight."

>> Click here to watch

3. Biden discusses his son's dealings with Ukraine.

Hours after ABC News published an interview with Biden's son, Hunter, about his business dealings with Ukraine, the elder Biden also addressed the issue on the debate stage.

"Look, my son did nothing wrong," Biden said, fielding a question from moderator Anderson Cooper over whether it was OK for Hunter to be involved in foreign businesses while Biden was vice president. "I did nothing wrong. I carried out the policy of the United States government in rooting out corruption in Ukraine. And that's what we should be focusing on."

He later added: "My son's statement speaks for itself. I did my job. I never discussed a single thing with my son about anything having do with Ukraine. No one has indicated I have. We've always kept everything separate. Even when my son was the attorney general of the state of Delaware, we never discussed anything, so there would be no potential conflict."

Biden continued that he was "proud of the judgment" his son made.

Neither Biden has been officially accused of wrongdoing.

>> Watch his response here

4. O'Rourke and Buttigieg disagree over gun buybacks.

O'Rourke reiterated his support for a mandatory buyback of AK-47 and AR-15 rifles, but when asked about how he'd enforce such a policy, he said he'd "expect [his] fellow Americans to follow the law."

"We don't go door to door to do anything in this country to enforce the law," O'Rourke said. "I expect Republicans, Democrats, gun-owners, non-gun-owners alike to respect and follow the law."

Buttigieg was not impressed.

"Look, Congressman, you just made it clear that you don't know how this is actually going to take weapons off the streets," Buttigieg said. "If you can develop the plan further, I think we can have a debate about it. But we can't wait. People are dying in the streets right now."

He added: "We cannot wait for purity tests. We have to just get something done."

O'Rourke dismissed Buttigieg's argument.

"This is not a purity test," O'Rourke replied. "This is a country that loses 40,000 of our fellow Americans every year to gun violence. This is a crisis. We've got to do something about it."

He went on to say lawmakers should listen "to those moms who demand action, to those students who March for Our Lives" and focus less on "polls and the consultants and the focus groups."

"The problem isn't the polls," Buttigieg responded. "The problems is the policy. And I don't need lessons from you on courage, political or personal."

>> Watch the exchange here

5. Harris presses for Trump Twitter ban.

Harris, who has previously called on Twitter to ban Trump's account, took aim at Warren for not supporting her plan.

Harris, who has previously called on Twitter to ban Trump's account, took aim at Warren for not supporting her plan.

"I just want to say that I was surprised to hear that you did not agree with me that on this subject of what should be the rules around corporate responsibility for these big tech companies, when I called on Twitter to suspend Donald Trump's account, that you did not agree, and I would urge you to join me," Harris told Warren, alleging that Trump has used the platform "to openly intimidate witnesses" and "obstruct justice."

"So look, I don't just want to push Donald Trump off Twitter," Warren replied. "I want to push him out of the White House. That's our job."

Harris didn't back down, responding: "Well, join me. Join me in saying that his Twitter account should be shut down."

"No," Warren said.

>> Click here to watch