House Majority Whip James Clyburn endorsed Joe Biden today, just three days ahead of South Carolina's Democratic primary-- a contest that could be make-or-break for the former vice president's presidential campaign.

"I've known for a long time who I was going to vote for. I'm voting for Joe Biden. South Carolinians should be voting for Joe Biden," Clyburn said at an event in North Charleston.

Clyburn tweetedabout his support for the former vice president, who joined him at an event in North Charleston.

"We don't need to make this country great again this country is great. That's not what our challenge is. Our challenge is making the greatness of this country accessible and affordable for all," Clyburn said.

Clyburn, 79, is one of the most popular and influential African American politicians in the state. With long-established ties to Biden, his support does not come as a surprise but rather suggests Biden needs all the help he can get to shore up support from black voters, who make up two-thirds of the Democratic electorate.

Biden's campaign is betting heavily on a South Carolina victory to carry him through to victories in the March 3 Super Tuesday contests. His performance so far in the early contests has been lackluster: fourth place in Iowa; fifth place in New Hampshire; and a distant second place in Nevada.

During Tuesday night's debate in Charleston Biden confidently declared: "I will win South Carolina." When pressed if he would continue his campaign if he doesn't come in first in the state's primary Biden repeated his prediction he would win.

Biden joined Clyburn at the Wednesday event and praised him as a "extraordinary leader" and said his reputation extends beyond South Carolina. "Jim has a voice of powerful, powerful moral clarity."

Without mentioning Vermont Sen. Bernie Sanders, the frontunner in the primary contest, Biden said there is talk about "revolution" but he argued, "what the country is looking for is results" and cited his role in passing Obamacare.

Virtually all polling in South Carolina has Biden leading, but his once seemingly insurmountable double-digit lead has eroded to single digits in the closing weeks of the race. The latest CBS News/You Gov poll has Biden leading the field with 51%, followed closely by Sanders with 44%.

An emotional Clyburn said he was "fearful" about the state of the country and pointed to Biden as the candidate to lead the party.

"I know his heart, I know who he is, I know what he is. This country is at an inflection point."

- NPR's Juana Summers contributed to this story

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