Detroit, Michigan (CNN) A handful of low-polling moderates hoped to break through in a crowded Democratic field during Tuesday's debate by confronting the top-tier candidates on stage, Massachusetts Sen. Elizabeth Warren and Vermont Sen. Bernie Sanders.

The two most progressive candidates in the 2020 Democratic field struck inspirational tones, with Warren urging Democrats to be "the party of big, structural change." And they won over the crowd as they debated with moderate critics who tried to question their electability and the feasibility of their ideas, but failed to knock either candidate on their heels even once.

In the process, they could have eased primary voters' fears that their policy proposals would make ripe targets for President Donald Trump and the GOP in a general election.

For their part, moderates pushed back as they tried to define themselves on health care and decriminalizing the border. Mostly, though, their highlighting of ideological differences within the party offered Warren and Sanders a tune-up for higher-stakes showdowns this fall against the Democratic front-runner, former Vice President Joe Biden.

Here are seven takeaways from Tuesday night, the first of the two nights of CNN's Democratic debate in Detroit:

1. Warren and Sanders swat away their critics

Former Maryland Rep. John Delaney's argument for pragmatism midway through the debate teed Warren up -- and she landed a haymaker.

"I don't understand why anybody goes to the trouble of running for president of the United States just to talk about what we really can't do and shouldn't fight for," Warren said.

The crowd erupted. Before the debate ended, Delaney's Wikipedia page had been updated to say he'd died at Warren's hands in Detroit.

It wasn't the only time Warren took on Delaney. Early on, she called his attacks on "Medicare for All" proposals "Republican talking points."

Then there was Sanders' retort when Ohio Rep. Tim Ryan told him that "you don't know that" as he questioned the coverage Medicare for All would provide.

"I do know. I wrote the damn bill," Sanders shot back.

The visuals were memorable, too. Sanders at one point threw his hands up at Hickenlooper. Warren rubbed her hands at the thought of implementing her 2% wealth tax on Delaney's $65 million personal fortune.

Delaney, Ryan, former Colorado Gov. John Hickenlooper and Montana Gov. Steve Bullock all went at Sanders and Warren from the right. Delaney began the debate by comparing the two to failed Democratic nominees George McGovern (1972), Walter Mondale (1984) and Michael Dukakis (1988).

The problem facing the moderates is that their arguments largely consisted of dire warnings about the political consequences of moving too far left. They didn't offer Democratic voters an alternative vision for a post-Trump America.

It's why none of them landed real blows on Warren or Sanders all night. If anything, sparring with the low-polling quartet served to sharpen Warren and Sanders for the fights against stronger opponents ahead.

2. No daylight between Warren and Sanders

The top two-polling progressives in the Democratic field were positioned on stage next to each other Tuesday night. But they showed no appetite for a fight with each other.

Instead, Warren and Sanders largely stood together, beating back moderate critics all night.

The two are courting different voters right now, but eventually, one of the them will need to consolidate progressive support to win the Democratic nomination.

Still, Tuesday night showed that the time to turn against each other could be months away. Both are considered top-tier candidates who poll viably and are raising money effectively, and both appear to believe it's far too early to take such a risk.

3. 'Dark psychic force'

Author Marianne Williamson provided one of the night's most memorable moments when she addressed the water crisis in Flint, Michigan, with a stirring condemnation of environmental racism -- and other candidates' approach to talking about it.

"This is part of the dark underbelly of American society, the racism, the bigotry, and the entire conversation that we're having here tonight -- if you think any of this wonkiness is going to deal with this dark psychic force of the collectivized hatred that this President is bringing up in this country, then I'm afraid that the Democrats are going to see some very dark days," she said.

"We need to say it like it is," Williamson said. "It's bigger than Flint. It's all over this country. It's particularly people of color. It's particularly people who do not have the money to fight back, and if the Democrats don't start saying it, why would those people feel they're there for us? And if those people don't feel it, they won't vote for us and Donald Trump will win."

The answer was a reminder of how powerful the perspective of a political outsider can be in presidential races. Williamson is a low-polling long-shot, but generated buzz with her condemnation of "wonkiness" on racism.

4. Seeking a middle ground on health care

The debate began with a battle over health care dominated by Sanders and Warren defending Medicare for All against Delaney, Hickenlooper and Montana Gov. Steve Bullock, who cast it as politically fraught in a general election.

Former Texas Rep. Beto O'Rourke and South Bend, Indiana, Mayor Pete Buttigieg sought middle grounds -- and their answers on health care underscored where they are trying to fit into the Democratic field.

O'Rourke touted a plan called "Medicare for America." It would enroll uninsured Americans in Medicare, and allow those who are dissatisfied with their private insurance to opt into Medicare -- while retaining private insurance for those who wish to keep it.

"Our plan ensures everyone is enrolled in Medicare or can keep their employer-sponsored insurance," he said.

Buttigieg argued for a similar approach -- and said Democrats should stop worrying about being called socialists by Republicans over the health care policies they back.

"If it's true that if we embrace a far-left agenda, they're going to say we're a bunch of crazy socialists. If we embrace a conservative agenda, you know what they're going to do? They're going to say we're a bunch of crazy socialists," he said. "So let's just stand up for the right policy, go out there and defend it."

5. Arguments for reparations

Asked about racism, O'Rourke was the first Democrat on stage to argue for a step toward reparations.

"The very foundation of this country -- the wealth that we have built, the way we became the greatest country on the face of the planet -- was literally on the backs of those who were kidnapped and brought here by force," he said.

O'Rourke said he backs legislation by Texas Rep. Sheila Jackson Lee that would create a commission to study reparations.

It was an effective moment for O'Rourke -- who, like Buttigieg, Williamson and Minnesota Sen. Amy Klobuchar -- didn't end up playing a part of the memorable clashes with other candidates, because they didn't fit into the progressives-vs.-moderates theme that Warren, Sanders and their critics established early on.

Williamson also had a moment as she defended her plan to offer $200 billion to $500 billion in reparations.

"We need to recognize when it comes to the economic gap between black and whites in America, it does come from a great injustice that has never been dealt with," she said.

6. Trying to make moments

Klobuchar struggled more to get into the action. She didn't attack Warren and Sanders the way others did, and there are reasons for her approach -- Klobuchar could end up a viable contender for the vice presidential nomination. But she did stake out moderate ground in her opening statement.

"You're going to hear a lot of promises up here, but I'm going will tell you this," she said. "Yes, I have bold ideas but they are grounded in reality. And, yes, I will make some simple promises. I can win this. I'm from the Midwest. And I have won every race, every place, every time."

Buttigieg's best moment came when he made the case for structural reform to the American political system -- the issue on which his proposals have been the furthest-reaching in the Democratic field.

"Of course we need to get money out of politics, but when I propose the actual structural democratic reforms that might make a difference -- end the electoral college, amend the Constitution if necessary to clear up Citizens United, have D.C. actually be a state, and depoliticize the Supreme Court with structural reform -- people look at me funny, as if this country was incapable of structural reform," Buttigieg said.

"This is a country that once changed its Constitution so you couldn't drink and changed it back because we changed our minds and you're telling me we can't reform our democracy in our time. We have to or we will be having the same argument 20 years from now."

7. Ideological split on decriminalizing the border

Mirroring the debate on health care, progressives and more moderate candidates split on the question of whether to decriminalize crossing the border illegally.

Warren said the current law "has given Donald Trump the tool to break families apart." Sanders also said he would decriminalize crossing the border.

But more moderate candidates said they would retain laws against crossing the border illegally.

"We can argue over the finer points of which parts should be handled by civil law and criminal law," Buttigieg said.

He later added: "If fraud is involved, that's suitable for the criminal statute. If not, it should be handled under civil law."

O'Rourke said he would waive green card fees, give so-called "Dreamers" -- undocumented immigrants who were brought into the US as children -- citizenship, ease the process of seeking asylum and aid struggling Central American countries.

"Then, I expect that people will come here, follow our laws, and we reserve the right to criminally prosecute them if they do not," he said.

Hickenlooper said: "I agree that we need to secure borders. There is no question about that. The frustration with what's going on in Washington is they are kicking the ball back and forth. Secure the borders and make sure whatever law we have doesn't allow children to be snatched from parents and put in cages."