Though former Vice President Joe Biden still leads the South Carolina primary race, according to the polls, his advantage has shrunk dramatically in recent weeks. And he blames his slide on a billionaire who has poured millions of dollars in the race for that state.

But he isn't talking about Michael Bloomberg, the billionaire media mogul who skipped campaigning in the Palmetto State to focus on the March 3 "Super Tuesday" races. It is former hedge-fund manager Tom Steyer whose spending Biden said is sapping his support.

"What's happening is you have Steyer spending hundreds of millions, tens of thousands of dollars, millions of dollars, out campaigning there," Biden said Sunday on CBS News' "Face the Nation" when asked why he thought his large lead had begun to erode in South Carolina, a state that is critical for his campaign after his loses in Iowa, New Hampshire and Nevada.

Biden said Steyer had spent "a lot of money" in South Carolina and he assumed that was "part of the reason why those numbers are down."

"But I don't know," he added.

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Steyer qualified for the Democratic debate, which will be held Tuesday in Charleston, South Carolina, after a CBS News poll released Sunday found him with 18% support among Democratic voters who are likely to vote in South Carolina's primary. He made the cut because it was the second poll to find him with at least 12% in the early voting state. Candidates could also qualify by winning at least one delegate in the earlier contests or getting at least 10% support in four national polls.

Joining Steyer on the debate stage will be Biden, Bloomberg, Sen. Bernie Sanders of Vermont, Sen. Elizabeth Warren of Massachusetts, former South Bend, Indiana Mayor Pete Buttigieg and Sen. Amy Klobuchar of Minnesota.

In the CBS News poll, Steyer trailed only Biden in South Carolina, who came in at 28%, and Sanders, who had 23%. Behind them were Warren (12%), Buttigieg (10%) and Klobuchar. Bloomberg was not included in the poll because he is not campaigning in the state, but 41% said they would consider him if he were on the ballot (59% said they wouldn't).

Biden's five-point lead over Sanders indicates a much tighter race in South Carolina than polling indicated in the first six months of the race. Most major polls found the former vice president with a 20-percentage point advantage into November. And no poll found him without a double-digit until a The Post and Courier survey in December found him with a 7-point lead over Sanders. Biden's average lead has slipped to five-percentage points, according to RealClearPolitics.

According to the CBS News poll, 91% of likely Democratic voters in South Carolina have seen Steyer's campaign ads. Biden was second at 70%, followed by Bloomberg at 68%, and Sanders and Warren at 62% each.

Biden has pinned much of his hopes for a South Carolina win on support from the state's large block of African-American voters. And he said they were the voters Steyer was targeting.

"A lot is happening in terms of the amount of money being spent by the billionaires to try to cut into the African-American vote. I think that has a lot to do with it," Biden said.

The poll found Biden was the top candidate for 35% of likely Democratic primary voters in South Carolina who are black, while Steyer was second at 24% and Sanders third at 23%.

Steyer said on "Fox News Sunday" that his support among black voters was about his message, not his spending.

"South Carolina happens to be a place that has a pretty high concentration of African-Americans. And those happen to be people that I talk to a lot and have a long history of working with," Steyer said.

He said he was the only remaining candidate "who is openly, consistently for reparations for slavery"

"I think one thing is I’m very willing to talk about race," he said. "I believe that there is a substantial racial subject in virtually every policy area in the United States."