WASHINGTON — Progressive Democrats are driving the conversation early on in the 2020 race with candidates looking to drive up enthusiasm with left-leaning grassroots activists and donors. But some in the party believe the early frontrunners have misjudged the party's base, leaving an opening for a candidate to offer a centrist correction.

Some look to a potential run by Vice President Joe Biden or the newly-announced former Rep. Beto O'Rourke as a possible vehicle for a middle-of-the-road campaign. They point to polls that suggest rank-and-file voters share their concerns that the party is moving too far to the left: A survey by Gallup in December found 54 percent of Democratic respondents wanted a "more moderate" party.

"I would say there's an energized moderate majority in the Democratic Party," Jim Kessler, senior vice president for policy at centrist think tank Third Way, told NBC News. "They're quiet compared to the activist voices on the left, and there's a difference between volume in decibels and volume in numbers."

But often lost in the discussion is what the term "moderate Democrat" even means at a time where many of the party's old ideological divides are collapsing and unexpected new ones are popping up. And there's still no declared presidential contender who has fully defined what the centrist alternative looks like for 2020.

So what makes a moderate Democrat?

Crossing enemy lines

The single biggest split might be how a Democratic candidate envisions working with the other side as president, whether that's Republicans or businesses and interest groups considered hostile to progressive causes.

On the one side, Sen. Bernie Sanders, I-Vt., and Elizabeth Warren, D-Mass., lead the populist pack with speeches that casts self-interested elites and their political allies as an oppressive force that must be defeated in order to accomplish real change.

On the other side, some candidates are testing out a more moderate message of cooperation.

This is a distinguishing approach for O'Rourke, who tacitly backed a Texas House Republican's re-election in 2018 and earned endorsements on Thursday from two moderate House Democrats.

"My sense is, following some success that I had in Congress, and working with Republicans to actually get things signed into law, including both President Obama and President Trump's administrations, that I may have an ability to work with people who think differently than I do, come to a different conclusion that I've come to on a given issue, and yet find enough common ground to do something better than what we have right now," O'Rourke said in a Vanity Fair cover-story profile published last week.

Former Colorado Gov. John Hickenlooper, a self-styled "extreme moderate" has played up his willingness to sit down with oil and gas companies executives to hammer out compromises on environmental policy.

The contrasting views of how to deal with opponents are already starting to come into conflict, with even formerly boilerplate nods to the other party now a topic of fierce debate.

After Biden praised his successor, Vice President Mike Pence, as "a decent guy," former New York gubernatorial candidate and actress Cynthia Nixon called him out on Twitter for "hollow civility" that glossed over Pence’s record on gay rights.

Biden retracted his comments, but has since made clear that he plans on emphasizing bipartisanship should he run.

In a speech to the Delaware Democratic Party on Saturday night he delivered an impassioned defense of finding "consensus" in politics even as he boasted he had "the most progressive record of anybody running...anybody who would run"

"We don’t demonize our opponents, we don't belittle them, we don't question their motive, we question their judgment, but not their motive, we don't treat the opposition as the enemy," Biden said. "We might even say a nice word every once and a while about a Republican when they do something good."

Gov't-run health care

When it comes to economic policy, Democrats are mostly running on the same broad ideas: Taxing the rich more to pay for investments in areas like health care, the environment and education.

But there are still big questions over how far they're willing to go in advancing these priorities, how they’ll pay for them and whether they lead with bold all-or-nothing proposals or take incremental steps en route to shared goals.

"I do think there are distinctions among the candidates on the question of ambition," said Heather McGhee, president of the progressive think tank Demos.

The most significant policy split so far has been about Medicare For All, where the Democratic field has disagreed over whether to maintain the current system of private employer-based insurance or scrap it entirely in favor of a single-payer Medicare For All plan.

Some Democrats have started to sketch the outlines of a "moderate" lane by rejecting single-payer on political and policy grounds.

On Friday, O'Rourke backed away from his support for full single-payer health care, which he had endorsed in his race last year against Texas Sen. Ted Cruz. "I'm no longer sure that's the fastest way for us to get there," O'Rourke told reporters, instead discussing a bill that would allow people to buy a public insurance plan or keep their employer-provided insurance.

Sen. Amy Klobuchar announces her run for president in Boom Park, Minneapolis, on Feb. 10, 2019. NBC News

Sens. Sherrod Brown, D-Ohio, who ultimately decided not to run, and Amy Klobuchar, D-Minn., who is running, both made news by not joining their colleagues in co-sponsoring Sen. Bernie Sanders. Medicare For All bill and instead arguing for an optional Medicare buy-in.

"It could be a possibility in the future," Klobuchar said of Medicare For All at a CNN town hall. "I'm just looking at something that will work now."

How big is too big

The progressive candidates, especially the senators in the race, have been eagerly trying to one-up each other with the most far-reaching policy proposals on taxes, education, housing and child care, many of which are priced in the trillions of dollars.

While candidates have yet to directly clash often, there are signs a moderate challenger might distinguish himself or herself by critiquing rival plans as too large, too costly or too risky.

In addition to Medicare For All, the Green New Deal is one area where there are signs of unease along these lines. Some candidates have suggested its goal of getting to net zero carbon emissions by 2030 is untenable.

"It's a very ambitious and maybe excessively ambitious goal," Hickenlooper said last week while praising its "urgency."

Former Rep. John Delaney, who is running for president. went further, calling it "about as realistic as Trump saying that Mexico is going to pay for the wall."

Fiscal conservatism is mostly on the outs in both parties, but some newly elected Democrats emphasized it in their midterm campaigns. It's possible the projected cost of plans could come into focus as a larger divide.

The unexpected...

While many issues like gay marriage and legalizing marijuana that distinguished centrist Democrats from progressives have disappeared, there are signs that future disagreements might take their place in 2020.

On many issues, there's little dispute. Pro-life Democratic politicians, once a significant faction, are nearly extinct. The new House majority has already passed gun safety bills and many new members in swing districts campaigned on the issue. On immigration, the party is largely united in opposition to Trump's agenda and in support of a path to citizenship for undocumented Americans. Party attitudes on race have also shifted as white Democrats have grown more likely to describe discrimination as a problem and endorse action to confront it.

"I think in the past, the cleave between the more progressive wing of the party and more centrist wing of the party was on a lot of social issues," Kessler said. "That doesn't exist anymore."

But as Democrats reach consensus on some issues, new splits keep opening up.

After a debate over financial reparations for African Americans broke out, for example, some of the field politely rejected the idea or tried to pivot to a broader discussion of inequality, while former Housing Secretary and 2020 candidate Julian Castro seized on the issue and challenged others to embrace it.

Similarly, when "Abolish ICE" emerged as a new slogan on social media last year, candidates parted ways over whether to reject or embrace it. Campaigning in New Hampshire, Sen. Kamala Harris, D-Calif., endorsed changing "Columbus Day" to "Indigenous Peoples' Day" in response to a town hall question from a voter. Several candidates recently backed ending the filibuster and expanding the Supreme Court, two ideas progressive activists have sought to push into the 2020 debate.

Then there was the recent blow-up over Rep. Ilhan Omar's comments on Israel in which Harris, Sanders, and Sen. Elizabeth Warren sided with the Minnesota Democrat's defenders in left-leaning activist circles, while Sen. Kirsten Gillibrand, D-N.Y. and Sen. Cory Booker, D-N.J. condemned the remarks while expressing concern about bigotry directed against her.

The candidates are likely to face plenty more tough calls — whether to impeach? — over which pressure campaigns demand their urgent attention, which are just passing noise and whether appeasing one restless group in the moment risks a backlash from a different one.