In a matter of 72 hours, Joe Biden parlayed a dominant victory in South Carolina into a steamrolling performance on Super Tuesday: He not only won substantially African American electorates like Alabama’s and Virginia’s, but he also carried Texas and scored huge coups by winning Massachusetts, Minnesota and Maine — all states thought to be favorable to Bernie Sanders.

And Biden did so without much of a personal, TV or field presence in any of them.

According to the latest NBC News projection, Biden leads Sanders by 513 to 461 in pledged delegates, with 105 for other candidates (1,991 are required to win the nomination). There are still millions of votes to count in California in the coming days, giving Sanders room to grow. But Biden’s total will also grow as his best states are certified and delegates are awarded based on the results calculated in each congressional district.

At first glance, Biden’s delegate lead doesn’t look that imposing. But there are three reasons Super Tuesday may have just given him a lead that could be extremely difficult for Sanders to erase by the July convention:

1. Many of the delegates Sanders won Tuesday are attributable to early votes cast before South Carolina voted and moderate candidates exited. The only states Sanders won Tuesday besides his home state, Vermont, were California, Colorado and Utah — all states where a huge share of ballots were mailed in before Biden turned the race on its head in South Carolina and Amy Klobuchar and Pete Buttigieg exited and endorsed him. (While California remained too early for NBC News to call, Sanders was leading there late Wednesday.)

Yet, despite those millions of ballots, Sanders is on track to finish well behind Biden in the Super Tuesday delegate count. And Sanders won’t have the advantage of huge pre-South Carolina early vote troves in any future contests. Even in Texas, where Sanders carried the early vote by a substantial margin, Biden was so dominant among Election Day voters that he carried the Lone Star state by 4 points.

2. The rest of the March calendar favors Biden, not Sanders. Sanders’ strongest groups to date have been young voters and Latinos, two groups that are overrepresented in the Southwest. But California and Texas have already voted, and Biden is poised to stretch his lead in older and more African American states such as Michigan, Missouri and Mississippi (March 10), Florida and Illinois (March 17) and Georgia (March 24).

3. Democratic voters could decide not to prolong the contest. In 2004, the last time Democrats were vying to beat an incumbent GOP president, John Kerry won a string of impressive February victories and primary voters decided to rally around the front-runner, effectively ending the race on Super Tuesday. This year, Democrats might be so desperate to close ranks to beat President Donald Trump that they increasingly coalesce around Biden.

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Next week’s key contest is Michigan. Biden should have large advantages in Missouri and Mississippi, two states Hillary Clinton carried in the 2016 primary season. But if Sanders can’t retain Michigan, a contest he won in 2016, it would be tough to imagine any path back.

How long Sanders remains in the race is up to him. But the all-proportional nature of Democrats’ delegate allocation is a net detriment to party unity: It makes delegate leads look more surmountable than they really are, which has the effect of giving trailing candidates false hope and protracting primary battles that are effectively already over. It happened in 2008 and 2016, and it may be about to happen again in 2020.

Even if Biden notches more victories throughout March, Sanders’ most fervent supporters may never be persuaded that a comeback is out of reach. But the reality is, for Sanders to catch up to Biden would require another seismic shift in the race.