Joe Biden's political firewall appears to be getting stronger, with the former vice president leading in South Carolina by 20 percentage points, fueled by strong backing from African American voters, according to a Monmouth University poll released Thursday.

The survey has Biden with 36% support from South Carolina Democratic primary voters, with Sen. Bernie Sanders of Vermont a distant second with 16% support. Worrying for the current front-runner Sanders, billionaire businessman Tom Steyer, who has been spending heavily in the state, is in a statistical tie with Sanders, with support from 15% of voters.

"Biden appears to be holding on to his core support among African Americans in South Carolina. The recent endorsement by Rep. James Clyburn should help solidify that," Patrick Murray, director of the independent Monmouth University Polling Institute, said in a statement.

The Road to Super Tuesday View All 32 Images

African Americans make up 60% of the Democratic primary vote in the Palmetto State, and Biden has argued that the eventual nominee must perform well among voters of color, an important part of the Democratic base. Lower turnout among African American voters in Michigan is one reason Democrat Hillary Clinton lost the state narrowly in the 2016 general election, analysts say.

The poll shows Biden claiming 45% of the African American vote. Steyer, notably, was the second choice for black voters, 17% of whom picked the climate change activist and businessman. Sanders wins 13% support among African American voters, Monmouth found.

Biden performed better than all of his competitors in the poll in every age, gender, race and income category, though the youngest age bracket Monmouth reported was 18- to 49-year-olds. Sanders has typically dominated among voters 18- to 29-year-old voters, an age group not singled out in the Monmouth poll.

Sen. Elizabeth Warren of Massachusetts got 9% support in the poll, followed by former South Bend, Indiana, Mayor Pete Buttigieg with 6%, and Sen. Amy Klobuchar of Minnesota with 4%. All are below the 15% threshold required to win pledged delegates.

Former New York City Mayor Michael Bloomberg is not on the South Carolina ballot, but 9% of voters said they would be "very likely" to vote for him if he were, and another 16% said they would be "somewhat likely" to cast a ballot for the self-funding billionaire candidate.

The poll was taken after the Nevada caucuses, which Sanders won by a landslide and with Biden a distant second but before the South Carolina debate, where Biden had one of his best debate nights and Sanders was on defense. More than three-fourths of those surveyed said the Nevada results had no impact on their choice.

Biden has been struggling in fundraising and finished poorly in Iowa and New Hampshire. But he has blossomed in South Carolina, and appears more confident and comfortable on the trail here.