A Reuters/Ipsos poll released just hours before Tuesday night's crucial debate in Charleston, South Carolina, shows that Vermont Senator Bernie Sanders has widened his lead for the Democratic presidential nomination and overtaken former Vice President Joe Biden in support among African Americans.

The poll could spell trouble for Biden, the one-time frontrunner who - until now - had been largely favored by the important voting bloc for the Democratic Party.

The latest poll showed support from African Americans rose 7 percentage points for Sanders in the last week, while it fell 10 points for Biden.

The former vice president has been banking on a strong showing in Saturday's South Carolina primary, a state where black voters make up more than half of the Democratic electorate.

But on Monday, Biden likely added to concerns about his viability at the South Carolina Democratic Party’s First in the South Dinner, where he mistakenly said he was running for Senate.

(SOUND BITE) (English) U.S. DEMOCRATIC PRESIDENTIAL CANDIDATE JOE BIDEN, SAYING:

"My name's Joe Biden. I'm a Democratic candidate for the Unites States Senate. Look me over. If you like what you see, help out. If not, vote for the other... Give me a look, though, okay?"

The Reuters/Ipsos poll, conducted from February 19th to the 25th, showed that - among all registered Democrats and independents - 26% said they would vote for Sanders, while 15% said they supported Biden.

The poll also showed billionaire media mogul Michael Bloomberg with 15%, down 2 percentage points since facing intense scrutiny in his first debate as a presidential candidate.

Massachusetts Senator Elizabeth Warren and former South Bend, Indiana, Mayor Pete Buttigieg were each supported by 10% of respondents. Another 4% said they would vote for Minnesota Senator Amy Klobuchar, and 3% said they were supporting billionaire philanthropist Tom Steyer.