Joe Biden has lost a substantial part of his black-voter support and is now running about even with Bernie Sanders, a new Wall Street Journal/NBC News poll has found.

The survey offered a caution sign for Mr. Biden just as the Democratic primary race moves into states with more racial diversity than Iowa and New Hampshire, two largely white states that held the first nominating contests.

Mr. Biden has long bet that his strong support among black voters would be the linchpin of his candidacy. Journal/NBC News surveys in 2019 found that about half of black voters who said they would participate in Democratic primaries named Mr. Biden as their top choice for nominee. In the new poll, both he and Mr. Sanders had support from just under one-third of black voters.

The poll underscored that many of the other Democratic candidates have a long way to go in winning support from black voters and not much time to do so, now that the pace of voting will soon accelerate. Black Americans make up about 60% of the party’s voters in South Carolina, which holds its primary Feb. 29, and they will represent a sizable chunk of the electorate in states that vote in March, including in the delegate-rich Deep South.

Nevada holds its caucuses on Saturday, marking the first racially diverse state to hold a nominating contest in this cycle.