Bernie Sanders’s campaign is making incredible strides, but huge hurdles remain.

On Feb. 18, a Fox News poll became the first national poll to show Sanders with a lead over Hillary Clinton. This week, Reuters published the second poll to show Sanders with a national lead.

There is tremendous enthusiasm among Sanders’s supporters, and their overall numbers appear to keep growing as time passes. But that’s one of the problems: He’s running out of time.

First, there are stiff structural barriers for his candidacy. As The New York Times reported this week, the way delegates are awarded — “based on vote tallies in congressional districts and some other areas” — favors Clinton over Sanders because she is more likely to win in those “delegate-rich districts.” And, as Politico reported this week, many of Sanders’s staunchest supporters, college students, will be on spring break when primaries are held in the states where they go to school. In fact, Politico’s analysis is that of the March 5-26 primary and caucus states, “more than half a million college students from 14 states will be on spring break at the same time that the presidential campaign train chugs onto their campuses.”

And then there are the primary calendar problems. The South Carolina Democratic primary — where Clinton is a heavy favorite to win — is Saturday. That contest will be followed three days later by Super Tuesday. Half the contests on that day will be in the South, which has not been particularly favorable in recent history to Democratic candidates not from that region, and where black voters who still heavily favor Clinton over Sanders make up a disproportionately large share of all Democratic primary voters.