Joe Biden Joe BidenSenate Republicans face tough decision on replacing Ginsburg What Senate Republicans have said about election-year Supreme Court vacancies Biden says Ginsburg successor should be picked by candidate who wins on Nov. 3 MORE’s once-outsize lead in the South Carolina Democratic presidential primary has narrowed to 5 points, according to a new Winthrop University poll, a sign of rising trouble for the former vice president in a state that his campaign has long considered a political firewall.

The poll still shows Biden leading the pack with 24 percent support among likely South Carolina primary voters. But that represents a significant decrease from a similar Winthrop University poll from September that showed the former vice president with a 20-point advantage in the state.

The latest poll also shows a surge for Sen. Bernie Sanders Bernie SandersKenosha will be a good bellwether in 2020 Biden's fiscal program: What is the likely market impact? McConnell accuses Democrats of sowing division by 'downplaying progress' on election security MORE (I-Vt.), who has gained considerable momentum since notching top finishes in the Iowa caucuses and New Hampshire primary earlier this month.

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The progressive firebrand sits in second place with 19 percent support in South Carolina, up from 8 percent in the previous September survey.

Only one other candidate, billionaire activist Tom Steyer Tom SteyerTV ads favored Biden 2-1 in past month Inslee calls Biden climate plan 'perfect for the moment' OVERNIGHT ENERGY: Trump administration finalizes plan to open up Alaska wildlife refuge to drilling | California finalizes fuel efficiency deal with five automakers, undercutting Trump | Democrats use vulnerable GOP senators to get rare win on environment MORE, registered double-digit support in the poll. He trails Sanders at 15 percent.

South Carolina, which holds its presidential primary Feb. 29, is the first state to vote in the nominating contest in which a majority of the Democratic electorate is black.

Biden has long cast the Palmetto State primary as something of a firewall for his campaign; a contest in which his strong support among black voters could help propel his presidential bid into Super Tuesday on March 3, even after middling finishes in Iowa and New Hampshire.

But the Winthrop University poll suggests that Biden’s early setbacks in the primary race may be eroding his momentum, even in a state that his aides have cast as rock-solid for his campaign.

Still, there are signs that the race in South Carolina remains fluid. While most respondents to the poll — 55 percent — said they are “very sure” who they will vote for, 43 percent said they might still change their mind ahead of the primary.

The Winthrop University poll surveyed 443 likely Democratic primary voters in South Carolina from Feb. 9-19. It has a margin of error of 4.7 percentage points.