Joe Biden Joe BidenBiden says voters should choose who nominates Supreme Court justice Trump, Biden will not shake hands at first debate due to COVID-19 Joe Biden should enact critical government reforms if he wins MORE is counting on his firewall of support in South Carolina to help him win the 2020 presidential nomination. But Democrats are warning that if the former vice president doesn't perform well in Iowa and New Hampshire, his lead in South Carolina could be in jeopardy.

In recent weeks, Biden has been losing ground in the first two early voting states, according to polling. And if he falls behind Sens. Elizabeth Warren Elizabeth WarrenOvernight Defense: Appeals court revives House lawsuit against military funding for border wall | Dems push for limits on transferring military gear to police | Lawmakers ask for IG probe into Pentagon's use of COVID-19 funds On The Money: Half of states deplete funds for Trump's 0 unemployment expansion | EU appealing ruling in Apple tax case | House Democrats include more aid for airlines in coronavirus package Warren, Khanna request IG investigation into Pentagon's use of coronavirus funds MORE (D-Mass.), Bernie Sanders Bernie SandersTrump, Biden will not shake hands at first debate due to COVID-19 Sanders tells Maher 'there will be a number of plans' to remove Trump if he loses Sirota reacts to report of harassment, doxing by Harris supporters MORE (I-Vt.) or South Bend, Ind., Mayor Pete Buttigieg Pete ButtigiegBillionaire who donated to Trump in 2016 donates to Biden The Hill's Morning Report - Sponsored by Facebook - GOP closes ranks to fill SCOTUS vacancy by November Buttigieg stands in as Pence for Harris's debate practice MORE, he will have to reassert his case to voters, Democratic strategists and political observers say.

“Bad losses in Iowa and New Hampshire, and some other states, could easily hurt Biden,” said Julian Zelizer, a professor of public affairs and history at Princeton University.

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So far, the former vice president has retained strong support from voters in South Carolina. A couple of recent surveys showed approximately one-third of likely voters in the Palmetto State plan to support Biden.

A Quinnipiac poll showed him leading Warren by 20 points, and a survey conducted by the University of North Florida showed Biden holding a commanding lead with 36 percent of likely voters, with both Warren and Sanders trailing by more than 26 points.

But strategists say that doesn’t mean those voters are locked in, particularly if other candidates begin to pick up speed between now and Feb. 29, when the South Carolina primary takes place.

"Savvy black voters will reassess their options," said Basil Smikle, a Democratic strategist who served as a campaign aide to 2016 presidential nominee Hillary Clinton Hillary Diane Rodham ClintonThe Memo: Trump furor stokes fears of unrest Bloomberg rolls out M ad buy to boost Biden in Florida Hillicon Valley: Productivity, fatigue, cybersecurity emerge as top concerns amid pandemic | Facebook critics launch alternative oversight board | Google to temporarily bar election ads after polls close MORE. "Every candidate will have to remake their case to, and reaffirm their policy prescriptions for, the African American community — Biden included."

Another strategist was more direct: “The support won’t be there if he’s slipping. There’s just no way.”

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But Biden allies say they’re on firm footing in the state and they don’t anticipate that changing, even if he comes in behind other candidates in one of the earlier states.

“The South Carolina firewall has proven pretty resilient because of his connection with African American voters,” one longtime ally said. “It endured the surge of other flavor-of-the-month candidates. It can certainly endure him finishing in a jumble of candidates at the top of the heap in Iowa and New Hampshire.”

“Blowout losses in one of those states would be the real test but it also matters who he’s losing to and how the rest of the field reacts to the results of Iowa and New Hampshire,” the ally added.

“If a boy mayor who has been famously unable to connect with African Americans is the hot hand coming out of Iowa, Biden’s support might even grow, because presumably by then [Sens. Kamala] Harris and [Cory] Booker will be out of the race themselves and their voters seem more likely to become Biden voters,” the ally said of Buttigieg, who has struggled to gain support among black voters.

Biden could also be bolstered by national polls, where he has continued to beat out his competitors. A CNN poll out on Wednesday showed him receiving 28 percent among Democrats and Democratic-leaning independents. He was followed by Sanders at 17 percent and Warren at 14 percent. Buttigieg, who climbed in the poll, came in at 11 percent.

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And political observers say there are other scenarios that could help Biden prevail in South Carolina, especially in a field that is still growing at this late stage in the primary season.

“A lot of water is going to pass under the bridge between now and February. Scenarios abound, especially if, as is likely, there are still a dozen or more candidates in the field when voting starts,” said Cal Jillson, a professor of political science at Southern Methodist University. “If two different people win in Iowa and New Hampshire and Biden is close and competitive, then South Carolina could buoy him up and send him into Super Tuesday with some momentum.”

Still Jillson acknowledged that if Biden finishes out of the top three in both of the first two events, “sentiments could shift quickly against him.”

In recent days, as Buttigieg has soared to the top of polls in Iowa and even one in New Hampshire, Biden has focused on Iowa “with renewed intensity,” Jillson said.

Biden’s campaign launched an ad this week in Iowa highlighting his experience and his ability to “stand with our allies and know them by their first names.”

Next week, he is set to begin an eight-day bus tour of the Hawkeye State, hoping to make up some lost ground and connect with voters as caucus day inches closer.

The Democratic strategist said Biden should devote more resources to Iowa and New Hampshire or he’ll be disappointed in South Carolina.

“One thing leads to another,” the strategist said. “You don’t win states in a vacuum.”