Progressive party factions push for Sanders' support as Warren exits, Biden surges "Too much at stake not to keep fighting," the Working Families Party said.

Those still competing in the 2020 presidential contest now stand poised for yet another round of votes; and as the field coalesces around former Vice President Joe Biden, his rival Sen. Bernie Sanders of Vermont now looks to don the progressive mantle solo, which until days ago, he shared with Massachusetts Sen. Elizabeth Warren.

Her exit from the race now leaves her supporters with a decision: join the coalescing chorus of party moderates surging to Biden’s centrist side; or, fall back on Sanders.

A clear ideological fork in the road has emerged for those who’d hoped Warren’s trail would carry her to the White House -- and a clear choice for her base still wanting a progressive nominee.

Monday evening the Working Families Party (WFP) held an organizing call with Warren's team and her supporters. ABC News was on the call as leaders discussed their path forward.

It comes the same day the WFP switched their support to Sanders, after endorsing Warren in September -- an enormous get at the time, after they had backed Sanders in 2016.

Now as Biden’s momentum looms large over the seven contests -- including six states' ballot boxes -- on Tuesday, the consensus was clear: time for mourning Warren’s loss must now go hand-in-hand with marshaling the progressive troops.

"It's been a rough week, not just for Warren supporters -- but for anyone who has been shut out or left behind," Maurice Mitchell, WFP’s national director, said. "But there is too much at stake not to keep fighting."

Kunoor Ojha, the states organizing director for the Warren campaign, described that out of the pain of their campaign ending, she had woken up with an "inexplicable adrenaline rush and urgency."

"A lot of people are voting tomorrow," Ojha said Monday night. "As painful as it has been to come to grips with the fact that our preferred candidate didn't make it -- I can take comfort in the fact that we do still have her strongest ideological ally in the race, and I want to make sure that we are all a part of his success."

The WFP's support of Sanders in 2016 -- and then their switch to Warren -- was met at the time with some controversy; their leaders downplaying that this might hint at a splintering of the progressive left. Now, their coming together behind Sanders once more may prove imperative.

Sanders’ success will now face fierce headwinds, as Biden rolls into the next contest jet fueled by his sweeping 10 victories on Super Tuesday. Moreover, statistical analyses and polling forecast Biden as a strong favorite to win at least four of the seven contests Tuesday night, including three of the four biggest delegate hauls.

The Vermont lawmaker, for his part, will look to reprise some of his 2016 wins -- as in Michigan, one of the largest delegate hauls Tuesday -- and drain some of Biden’s current surge. But he’ll need all the help he can get from those fresh pockets of support potentially offered by orphaned Warren Democrats.

Warren’s endorsement since her exit has been conspicuous by its absence: amid a flood of erstwhile 2020 candidates rushing to Biden’s side. Warren has yet to tip her hand in supporting either -- even as Sanders now must struggle to consolidate multiple progressive factions.

Still others, like the closely Warren-aligned group Progressive Change Campaign Committee, have been urging their nearly 16,000 members in Michigan to support Sanders. Their reasoning, however, is not as simple as Sanders’ role as last-standing progressive standardbearer: rather, they call such support ‘strategic’ in order to keep the race going and ensure both candidates are “tested" and can “go the distance." The idea being, a source with knowledge of the group’s thinking tells ABC News, to make both Biden and Sanders build up their muscles prior to the general.

"It's incumbent on all of us to fight like hell in order to elevate the Sanders campaign," Mitchell said on Monday night’s call. "And we're going to leave everything on the battlefield."

But that battlefield takes shape as the field narrows -- the progressive wing now looking to draw sharper contrasts with what Biden and Sanders can offer, both framed in the context of the void Warren leaves behind.

Mitchell called Sanders a "leading mouthpiece for issues critical to working families," listing off his support of the Green New Deal; universal childcare; health care and "taxing the ultra wealthy to invest in public good." He added that "like Warren," Sanders has an aggressive anti-corruption agenda and would "harness the power of government to tackle structural inequality."

"On the other hand, Joe Biden has often put the interests of the powerful and privileged ahead of the needs of working people," Mitchell went on -- not holding back from offering a laundry list of reasons he said Biden would be the wrong choice.

"[Biden's] bankruptcy bill for example, favored big banks and credit card companies over debt-strapped working families. His crime bill locked away a generation of black folks. His vote for the Iraq War cost millions of lives. And Biden's policies on health care and climate fail to address the scale of the crises working families on the planet face," he said. "Finally, Biden's vision of a post-Trump bipartisan utopia is at odds with reality, and lessons of the Obama administration."

He added, "We believe that the best way to beat [Trump] isn't with milquetoast corporate-friendly policies -- but with bold solutions that meet the needs of working people."

In the crowded primary contest that has pitched to and fro with highs and lows for many of the candidates, these next few votes may indicate a coming climax, as the field winnows even more and Democratic voters weigh their choice of who will go toe-to-toe with President Donald Trump in November.

Grieving Warren's loss in the race, and their loss of her as the final female front-runner; WFP now looks to Sanders.

"We 100% believe that Bernie Sanders is the best hope to move Warren's policy ideas forward," Mitchell said. "And we also know this: women's leadership isn't just a 'Nice to Have.' It really matters. We shouldn't have to wait decades more to see a woman president."

He added, "The next few months will be critical. And we'll need all the help we can get … onward."

ABC News' Adam Kelsey, Averi Harper and Cheyenne Haslett contributed to this report.