With three Democratic primary contests (mostly) settled, and a critical fourth slated for Saturday in South Carolina, the race for president has become a battle not just to unseat Donald Trump, but a reckoning within the party over which candidate can usher in the next era of Democratic dominance in Washington by winning up and down the ballot in November.

At the center of that reckoning is Sen. Bernie Sanders, I-Vt., the newly-minted Democratic front-runner, whose more moderate rivals argue will hurt the party’s chances of holding their majority in the U.S. House and winning back power in the U.S. Senate, and blunt any chance of passing the progressive agenda they have been preaching for over a year on the campaign trail.

That anxiety, described by one of former Vice President Joe Biden’s top surrogates as “down ballot carnage” has only increased as Sanders took an early lead in the delegate race.

After two decisive wins in New Hampshire and Nevada for Sanders, moderate Democrats from Biden, to former New York City Mayor Mike Bloomberg, Minnesota Sen. Amy Klobuchar and former South Bend, Indiana, Mayor Pete Buttigieg, are grappling with the prospect of his nomination and have since sought to cast the Vermont senator’s platform and ideology as far too liberal for the party.

“Let’s talk about progressive,” Biden said at the Democratic debate in Charleston earlier this week. “Progressive is getting things done…We got a lot done.”

“Look, the idea that there’s going to be this revolution, Americans aren’t looking for a revolution,” Biden said on the trail on Thursday in Conway, South Carolina, hitting Sanders for his calls for a political revolution. “They’re looking for progress.”

In the hopes of slowing down his momentum, the more moderate candidates have been criticizing the self-described Democratic Socialist for potentially bruising Democratic candidates further down the ticket in recent weeks.

For his part, Sanders has pushed back against the notion, arguing that his presidential campaign’s efforts to court new voters could increase turnout and have a positive effect on down ballot races.

“When you have a large voter turnout, which is, what I think our campaign is going to generate. It’s going to help everybody, from the top of the ticket to the bottom of the ticket,” he told reporters earlier this month.

Questions of electability turn to down-ballot viability

For Biden, who seeks to consolidate the middle as Democrats’ safest bet, he’s up against a fractured primary that appears to be brimming with moderates and a progressive base that is coalescing around Sanders.

“Do you think running as a socialist would help you in Georgia, help you in North Carolina, help you in South Carolina, help you in Texas?” Biden asked rhetorically during an interview on CNN Friday morning. “I’ll let everybody else make that decision.”

During Tuesday’s debate, Buttigieg, too, questioned Sanders’ ability to safeguard Democrats’ majority in the House, contending that his agenda conflicts with most of the party, particularly down-ballot candidates.

“If you want to keep the House in Democratic hands, you might want to check with the people who actually turned the House blue – 40 Democrats who are not running on your platform,” Buttigieg said at the debate. “They are running away from your platform as fast as they possibly can. I want to send those Democrats back to the United States House.”

Shortly after Buttigieg argued that many of the 40 House Democrats that helped secure the majority don’t want Sanders at the top of the ticket, Bloomberg also touted his financial support for those candidates.

“They talk about 40 Democrats,” the billionaire began. “21 of those were people that I spent $100 million to help elect. All of the new Democrats that came in, who put Nancy Pelosi in charge, and gave the Congress the ability to control this president, I b– got them.”

“If you keep on going, we will elect Bernie. Bernie will lose to Donald Trump…and the House and the Senate and some of the state houses will all go red,” he later argued.

Sanders countered, arguing the diverse political movement he leads offers a path to victory, even if his competitors argue otherwise.

“If you want to beat Trump, what you’re going to need is an unprecedented, grassroots movement of black and white and Latino, Native American and Asian people who are standing up and fighting for justice. That’s what our movement is about,” Sanders said.

Overshadowed by the presidential race, House Democrats face the not insignificant challenge of keeping their majority that includes 31 members representing districts won by President Trump in 2016. In the Senate, Democrats have an opening, needing a net gain of just four seats to take back control in a cycle where the GOP is defending 23 incumbents, including two in states won by Hillary Clinton, Colorado and Maine.

Among the four contenders, Biden’s support among congressional endorsements tops the scoreboard, with nearly 50, reflecting his down-ballot appeal. His backers include the highest-ranking African American member of Congress, House Majority Whip Jim Clyburn, along with nine Frontline Democrats, the most vulnerable House Democrats up for re-election in 2020, such as Congressman Colin Allred, D-Texas, and Sen. Doug Jones, D-Ala., one of the most endangered Democratic senators in 2020.

The only candidate close to Biden’s grip on congressional endorsements is Bloomberg, who entered the primary race late last year but has since clinched 23 congressional endorsements since November, including from seven Frontliners.

Allred’s support for Biden, he told ABC News, stems from the former vice president’s ability to “heal the country” with “empathy” after an “extremely divisive four years.” The freshman congressman is one of few Frontliners to weigh in on the primary, and as co-president of his class, he’s been encouraging others to do the same.

In 2018, Allred emerged as only one of two Texas House Democrats to flip a seat across the state, toppling incumbent GOP Congressman Pete Sessions to capture the suburban Dallas district. Hillary Clinton only narrowly carried the district over Trump four years ago, and Allred is expected to face a competitive re-election in 2020. But he is eager to run alongside Biden, who he said “appeals to the same voters that we appealed to in 2018,” referring to the 2020 Frontliners, who he called the “best politicians in the party right now” because they know “how to win.”

But the civil rights lawyer and former NFL linebacker said after 2018 he’s “surprised” by the party’s indecisiveness over the best pathway forward to defeat Trump.

“I think we have to learn the lessons of 2018, and I’m kind of surprised at least in the uncertainty about how do we win in this era when we just won the last election,” Allred said in an interview Friday. “We did it in a way that was welcoming to Republicans who felt like this version of the Republican Party didn’t represent them and the Independents who were looking for a home. I think that’s what we should do in the presidential election.”

Fears of Sanders leading the ticket: ‘Bernie is about Bernie’

The current Democratic front-runner, who does not have the backing of any Frontline Democrats – but much like his base, he has cemented support among the progressive wing in Congress – is contending with a party apprehensive to his policies and hesitant to fully embrace his rigid style of politics.

Allred would only say it is “very early” when asked about supporting the Democratic nominee, regardless of who it is, before adding, “You don’t know who it is going to be and I’m working to make sure Joe Biden is the nominee.”

A Democratic strategist who works on House races told ABC News that House Democrats are concerned over whether Sanders can be a “positive force for down-ballot Democrats” in part because “they haven’t seen it yet.” Some down-ballot Democrats believe “Bernie is about Bernie” because “he is not a member” of the Democratic Party, the strategist said.

But at the core of Democrats’ anxieties, regardless of Sanders’ polling in key battlegrounds or in hypothetical general election matchups against Trump, the strategist said, is that candidates don’t want “self-inflicted complications.”

“The socialism piece is a big problem and I think that people are worried that you know that they’re going to have to spend a considerable amount of time explaining away Bernie Sanders’ socialism and they just don’t have the resources to do that,” the strategist said. “It really potentially disrupts our healthcare message, which we know works.”

“It takes them off-message…Anytime you’re off-message, anytime you’re explaining, you’re probably losing,” the strategist continued, before adding that some Democrats believe Sanders would be “toxic in their districts.”

More moderate outside groups linked to Democrats, including pro-Israel lobbying group Democratic Majority for Israel’s political arm, and the Big Tent Project, a newly formed dark money group linked to Sen. Joe Manchin’s former aide, have taken up the anti-Sanders campaign on air, running seven-figure television ad campaigns taking aim at Sanders’ socialist label and questioning his electability in the general election.

“We have a lot of House members, potential House members, a lot of senators and potential senators running in difficult states and difficult districts,” DMFI PAC President Mark Mellman told ABC News. “And frankly, having somebody like Sanders, a self-proclaimed socialist, at the top of the ticket, whether you like those ideas or you don’t like your ideas, the reality is that makes it much more difficult for people running for Senate and House in difficult states and districts.”

Bernie’s backers tout his bonafides down-ballot

Some of Sanders’ most prominent proponents argue fears of the insurgent, two-time presidential hopeful winning the nomination are not just overblown, but discount the Vermont senator’s ability to both draw new voters into the political process, and appeal to working-class voters who have strayed from the Democratic Party in the era of Trump.

They say that Sanders’ showing in the first three contests, winning the popular vote in all three despite coming up short in the delegate battle out of Iowa, demonstrates his ability to assemble the diverse coalition needed to beat Trump.

“To say that somebody with those values of the top of the ticket is going to hurt them, is to say that having that coalition of people is going to hurt. That’s where the energy is, with young people, people of color, folks that have been excluded in many ways from the political process are now being included to serve above them,” Paco Fabian, the director of campaigns at Our Revolution, a political nonprofit that spun out of Sanders’ 2016 presidential campaign, told ABC News.

Supporters also say that he could be beneficial for the party since his strong showing with Latino voters in Nevada demonstrates his ability to appeal to and turn out a constituency key to both keeping the House and winning back the Senate.

“The Democratic universe has been talking a huge game for the last decade and a half now about how that constituency [Latinos] is the future of the party. Well, that future is here. And if the party chooses to see this, we will not only win for Sanders in the primary. He will be Donald Trump in November,” Derrick Crowe, a spokesperson at People’s Action, a progressive group boosting Sanders, told ABC News.

“If anything, Sanders at the top of the ticket will lead to an even better performance because the working-class communities of color have not participated in the voting process, especially at this stage in the election cycle,” Progressive Democrats of America’s Executive Director Alan Minsky told ABC News. “In amongst American Latinos, working-class Latinos, and now suddenly there’s a campaign that is incredible energy.”

Natalia Salgado, chief of civic engagement at the Center for Popular Democracy, said down-ballot concerns come from a “traditional political party gene” who “just want our reliable folks who are going to come out, who aren’t going to really sort of push us on the issues, who are okay with the status quo.”

“I think this is just a symptom of the Democratic Party feeling as if it’s under attack by all of these different organizing cells across the country and by candidates that are inspiring a bigger group of people,” Salgado told ABC News.

The Democratic strategist acknowledged Sanders’ competitiveness right now, but noted that the candidates are still in the early stages of the election, with a grueling general matchup against Trump still to come.

“The question is how does he stand up after months of negative advertising and what, if anything does he do to tweak his proposals so that…they could be a little more palatable for Democrats who are running in tough places,” the strategist said.

Despite the onslaught of criticism from his rivals since his rise in the polls, Sanders is adamant he is the one to beat.

“Take a look at some of the battleground states, Michigan, Pennsylvania, Wisconsin. Bernie beats Trump. Take a look at a poll that just came out this morning in Pennsylvania: Bernie is the only Democrat to beat Trump in Pennsylvania,” he told supporters in Winston-Salem, North Carolina, earlier this week. “So don’t tell me, Bernie can’t beat Trump. We’re gonna beat him. We’re gonna beat him here in North Carolina. We’re gonna beat him all over this country.”