South Carolina, where the majority of Democrats are African American, has historically served as a test for which candidates can appeal to black voters. Buttigieg, who performed well in both Iowa and New Hampshire, has struggled to connect with black Democrats, and polled at just 4 percent among them in the NBC/Marist poll.

Biden polled the highest with black voters at 35 percent, whereas Sanders sits at 20 percent. The former vice president is also expected to capture House Majority Whip Jim Clyburn's endorsement later this week, another boost for the his campaign in South Carolina.

Since he entered the race last year, South Carolina has been a stronghold for Biden. But Sanders has continued to close in, driven mostly by voters under age 45 (40 percent) and voters who describe themselves as progressive (34 percent).

Consistent with past trends, the poll shows that Biden performed best among voters who say they are moderate, at 38 percent, and voters who are older than 45, with 34 percent.

The poll was conducted before Sanders' domination in the Nevada caucuses — and partially before last week's debate in Las Vegas.

As the candidates approach Super Tuesday next week, some party leaders fear Sanders might cruise to the nomination if the moderate candidates continue to split the vote. South Carolina's results could narrow the field — or present another example of a divided party split among the crowded field of candidates.

The NBC News/Marist poll surveyed 539 likely Democratic primary voters from Feb. 18-21.