Fourth Democratic Presidential Debate: 5 Moments That Mattered

— -- If there was ever a time for the Democratic presidential candidates to sharpen their attacks on each other, Sunday was it.

The final debate before the Iowa caucuses between Hillary Clinton, Bernie Sanders and Martin O’Malley laid bare differences over gun control, health care and taking on Wall Street.

Here are five moments that mattered at Sunday night's debate, hosted by NBC News and YouTube and held in Charleston, South Carolina:

1. Clinton and Sanders Go Head-to-Head Over Health Care

In one of the most fiery moments of the night, Hillary Clinton and Bernie Sanders went head to head over health care and Clinton's accusation that Sanders wants to dismantle Obamacare.

She said she is "absolutely committed to universal health care," adding, "I certainly respect Sen. Sanders’ intentions. But when you’re talking about health care, the details really matter."

"We finally have a path to universal health care," Clinton said. "I don’t want to start over again with a contentious debate."

Sanders responded that it's "nonsense" to suggest he would undo the Affordable Care Act. "I helped write it, but we are going to move on top of that to a Medicare for all system."

Just two hours before tonight's debate, Sanders released his own health care and tax plan. Tonight when being asked about the plan, moderator Andrea Mitchell described it as a "very detailed plan." In an unusual response he tried to downplay his own policy proposal saying, "It's not all that detailed."

2. Clinton and Sanders Spar Over Gun Policy

Clinton and Sanders have feuded over gun policy on the trail in early states -- and their battle extended onto the debate stage Sunday night.

"Well, I think Secretary Clinton knows what she says is disingenuous," said Sanders, citing Clinton’s attacks. "I have a D-minus voting record from the NRA."

"I have made it clear, based on Sen. Sanders' own record that he has voted with the NRA, with the gun lobby numerous times," Clinton replied, engaging one of the few issues on which she can attack Sanders from a more liberal position.

3. Wall Street Makes It to the Debate Stage

It was a topic bound to come up: Wall Street and the big banks.

Sanders started with a dig at Clinton, saying: "I don't get personal speaking fees from Goldman Sachs."

Clinton hit back, saying the Vermont senator has "criticized" the president for "taking donations from Wall Street," accusing him of questioning the president's leadership.

"President Obama led our country out of the Great Recession," Clinton said. "I'm going to defend President Obama for taking on Wall Street, taking on the financial industry, and getting results."

Sanders said he worked for the president's elections, calling the two "friends."

Clinton called Sanders' kind words for Obama now "a little strange given what you said about him in 2011," when he said it would be a good idea for Obama to have a primary challenger for his re-election.

4. Sanders Channels Trump With Citing Poll Numbers

"Let me talk about polling," Sanders said in a moment that was reminiscent of GOP frontrunner Donald Trump. "Secretary Clinton well knows, when this campaign began, she was 50 points ahead of me. We were all of 3 percentage points. Guess what? In Iowa, New Hampshire, the race is very, very close."

Sanders continued: "In terms of polling, guess what, we are running ahead of Secretary Clinton in terms of taking on my good friend, Donald Trump. Beating him by 19 points in New Hampshire, 13 points in the last national poll that I saw."

5. Sanders Lashes Out Over Bill Clinton Question

After calling President Bill Clinton's previous scandals "disgraceful," Sanders lashed out over his frustration trying to focus his campaign on the issues.

"I cannot walk down the street, Secretary Clinton knows that, without being told how much I have to attack Secretary Clinton," Sanders said.

"I'm going to debate Secretary Clinton and Governor O'Malley on the issues facing the American people -- not Bill Clinton's personal behavior," he said.