Since the beginning of the Democratic nomination fight, Sanders has trailed Hillary Clinton among non-white Democrats by a wide margin. During the late summer, when it looked like Joe Biden might throw his hat into the ring, the non-white vote was split between Biden and Clinton. But when Biden decided not to run, that vote swung back to the former secretary of state, where it has largely stayed.
That doesn't matter in Iowa, where four percent of the 2008 Democratic turnout wasn't white. And it especially doesn't matter in New Hampshire, where the non-white vote was half of that. But it will matter, a lot, in a lot of big states that arrive shortly afterward on the electoral calendar.
Sanders's team has argued that it doesn't need to win with non-white voters, which is true. It has also argued that as voters get to know the senator better, his numbers will improve -- also true. But Sanders has had a long time to make his case in Iowa and New Hampshire, time that he won't have in states further down the line. In South Carolina, which votes on Feb. 27, the most recent big poll in the state (from Fox News last month) had Clinton up by 44 points.
Winning isn't everything; Sanders can still pick up delegates in states he loses. But losing badly in big states (like Texas) will give Clinton far more delegates than Sanders will earn with a dominant performance in, say, New Hampshire. Which, no matter what happens in the first two contests, is a big problem for the senator.