This raises a straightforward challenge for Mr. Sanders. He has nearly no chance to do as well among nonwhite voters as Mr. Obama did in 2008. To win, Mr. Sanders will need to secure white voters by at least a modest margin and probably a large one. In the end, Mr. Sanders failed to score a clear win in a state where Mr. Obama easily defeated Mrs. Clinton among white voters.

Mr. Sanders’s strength wasn’t so great as to suggest that he’s positioned to improve upon national polls once the campaign heats up. National polls show him roughly tied with Mrs. Clinton among white voters, and it was the case here as well. It suggests that additional gains for Mr. Sanders in national polls will require him to do better than he did in Iowa, not that the close race in Iowa augurs a close one nationally.

Mr. Sanders will have another opportunity to gain momentum after the New Hampshire primary. He might not get as much credit for a victory there as he would have in Iowa, since New Hampshire borders his home state of Vermont. But it could nonetheless give him another opportunity to overcome his weaknesses among nonwhite voters.