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 those in Alabama and Virginia, but he carried Texas and scored huge coups by winning Massachusetts, Minnesota and Maine — all states thought to be favorable to Bernie Sanders. And he did so without much of a personal, TV or field presence in any of them.

According to the latest NBC News estimate, Biden leads Sanders 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 current delegate lead doesn't look that imposing. But there are three reasons why Super Tuesday may have just given him a delegate lead that could be extremely difficult

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