Former Vice President Joe Biden is only 5 points ahead of Sen. Bernie Sanders (I-VT) in the latest poll of voters in South Carolina, the state where Biden had built a “firewall” of support among African-Americans.

The Charleston Post and Courier reported Sunday: “Biden, who once led by as much as 31 percentage points in South Carolina, holds a 5-point edge over Vermont Sen. Bernie Sanders.” The margin of error is 4 points.

The new poll result raises the possibility that Sanders could sweep all four of the early primary states that vote in February. He leads in the latest polls in Iowa and New Hampshire; he is statistically tied with Biden in the most recent poll in Nevada; and now he is clinging to a slim lead over Sanders in South Carolina.

It is not clear why Biden is fading, but Sanders has surged over the past several weeks, especially after the entire field of candidates failed to impress during the last Democratic debate before the Iowa caucuses.

The impeachment trial of President Donald Trump has kept Sanders, and other Senators running for the nomination, in Washington. However, it has also highlighted alleged conflicts of interest for Biden.

The Post and Courier poll also reported that billionaire left-wing donor Tom Steyer is now in third place in South Carolina, at 18 percent. CNN reported last month that Steyer had bought over 90% of the television advertising time in both South Carolina and Nevada, flooding the airwaves and possibly hurting Biden.