The fight over the definition of "progressive" dominated the first half of Thursday's debate between Hillary Clinton and Bernie Sanders on MSNBC, the first head-to-head debate between the two. It came just days before the crucial New Hampshire primary.

Here are seven moments that stood out:

1. "A progressive is someone who makes progress."

The debate focused on a central question about what it means to be a Democrat in 2016.

"A progressive is someone who makes progress," Clinton said.

"In my mind, what we have got to do is wage a political revolution," Sanders countered.

The shift toward debating the meaning and embodiment of progressivism comes three days after Clinton narrowly edged Sanders in the Iowa caucuses. Clinton and Sanders are battling now in New Hampshire, which holds a Feb. 9 primary. Sanders has enjoyed a double-digit advantage in recent polls, but Clinton is campaigning hard in the state that delivered her a victory in her 2008 campaign.

The debate started with an antagonistic tone after the Vermont senator levied an attack on Clinton via social media on Wednesday that said she is too moderate to represent Democrats today.

"You can be a moderate. You can be a progressive. But you cannot be a moderate and a progressive," the Sanders camp tweeted.

The Clinton camp volleyed back on Twitter: "This shouldn't be a debate about who gets to define 'progressive' — it should be about who will get real results for American families."

Clinton attacked Sanders on Thursday for opposing efforts to tighten gun laws and for suggesting that President Obama is also not liberal enough.

2. "Do I think President Obama is a progressive? Yes, I do."

During the debate, Sanders was pressed on whether he thinks President Obama is a progressive.

"Do I think President Obama is a progressive? Yes, I do," Sanders said. "I disagree with him on a number of issues including the trade agreement. But, yes, I think he has done an excellent job."

Sanders also, though, defended his attack on Clinton because she, at a recent event, described herself as a "moderate."

"Cherry-picking a quote here and there doesn't change my record," Clinton said.

The attack clearly hit a nerve in the Clinton camp. Shortly before Thursday's debate, the campaign released a new ad that features her saying, "We've been fighting the progressive fight for years."

While the Republican Party has been engaged for years between its establishment and conservative wings on what it means to be a Republican, Democrats have not had the same level of intraparty squabbling about what it means to be a Democrat.

The MSNBC debate saw Clinton and Sanders re-litigating the political disagreements that have percolated among Democrats for years.

3. Sanders defends opting out of the public-finance system.

On campaign finance, Sanders defended his decision to opt out of public campaign financing while campaigning for support of the publicly funded elections.

"We looked at it. It turns out to be a disaster," Sanders said, noting that the system is not equipped to deal with modern campaign-finance laws.

4. "The business model of Wall Street is fraud."

Sanders criticized Clinton for her relationship with Wall Street, including receiving high speaking fees for addressing groups that included Goldman Sachs.

"Wall Street is an entity of unbelievable economic and political power; that's a fact," Sanders said. "I believe that corruption is rampant."

He added, "The business model of Wall Street is fraud."

"We both want to rein in the excesses of Wall Street," said Clinton.

5. Back to Iraq: "A vote in 2002 is not a plan to defeat ISIS."

On foreign policy, Sanders also highlighted Clinton's vote in support of the Iraq War. The issue hobbled Clinton in the 2008 presidential campaign against Barack Obama. Sanders voted against the Iraq War.

Clinton tried to shift the focus to the modern-day threat posed by the Islamic State. "A vote in 2002 is not a plan to defeat ISIS," she said.

For all of the disagreeable talk, the two Democrats agreed against committing ground combat troops back to the Middle East to address the terrorist threat.

On the whole, the philosophical divide this year between Clinton and Sanders is not as deep as their stylistic and rhetorical differences. Both Democrats, for instance, voiced opposition to privatizing the veterans health care system.

Clinton and Sanders remain divided on the death penalty: Clinton supports it; Sanders does not. And while Clinton, like Sanders, opposes an outstanding multilateral trade pact, Sanders criticized her for coming later to that position than he did.

For Democrats, this primary race is often cast as a debate between the head — embodied by Clinton's pragmatism — and the heart — embodied by Sanders' uncompromising passion for liberal ideas.

6. "100 percent confident" emails won't be a problem in general election.

Clinton further defended herself against concerns that her use of a private email server as secretary of state would result in more serious charges against her. "I am 100 percent confident," she said.

Clinton has apologized for the private server, but maintains there was no wrongdoing. An ongoing federal review, however, revealed that some of the emails she received included classified information.

The candidates ended the sharp, two-hour debate on a conciliatory note.

7. VP Sanders? Maybe not. But "first person I will call."

Clinton dodged whether she would consider running with Sanders on the national ticket with a compliment. "If I'm so fortunate as to be the nominee, the first person I will call to talk to about where we go and how we get it done will be Sen. Sanders," she said to applause.

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