Bernie Sanders may have momentum. He may have overwhelming support among young Democrats. He may even have pulled rival Hillary Clinton left, in his direction. “Saturday Night Live” spoofed this on March 12 with a skit that showed Mrs. Clinton literally morphing into Senator Sanders as she spoke.

But he needs much more than that to actually win the Democratic presidential nomination. He needs to dominate upcoming primaries. He needs grand slams, hat tricks, slam dunks, and lots of other sports analogies for individual achievement.

He needs delegates, and lots of them. Fast.

Here’s the basic Sanders problem: Clinton has already run up the score in a number of Southern primaries. That’s because their Democratic electorates are heavily African-American, a demographic which has resisted Sanders’s appeal.

That’s allowed her to corral lots of delegates. In Mississippi last week, for example, Clinton won an astounding 83 percent of the vote. That awarded her 30 delegates. Sanders got four.

In pledged delegates (the kind you get from primary votes) Clinton leads Sanders at the moment by 772 to 551. That puts her about one-third of the way to the 2,383 delegates needed to win the nomination at the Democratic National Convention in Philadelphia.

We can hear you already – 772 to 551? That doesn’t seem so daunting. It’s a delegate deficit of 221. If Sanders surprises Clinton on March 15, winning Ohio and maybe Illinois and even Missouri, he should be right back in the game.

But he probably wouldn’t be. That’s because he’s facing a structural problem. Democratic primaries are proportional. In other words, delegates are awarded in proportion to the vote. If you win 51 percent, you get 51 percent of the delegates. And so on.

That’s different from the Republican primaries. The GOP has now reached a date following which it allows winner-take-all contests. If Donald Trump wins 51 percent of the Florida vote, he’ll get 99 Sunshine State delegates. His opponents will get 0.

Rules matter. If Sanders was running under GOP-style rules, he’d have lots of opportunity to make up his deficit in a bound. But under Democratic-style rules, the loser picks up delegates too. It’s harder to close the distance.

That’s why Sanders needs to run up the score. He needs many more upsets of a larger scale than his Michigan win to get close to Clinton in pledged delegates. Figuring the math, he needs an average victory margin of eight points in every upcoming state, according to Vox. And that includes big states such as Florida (heavily Hispanic) and New York (where Clinton was a senator).

Sanders just isn’t going to win all the remaining states. So, to reach that average of 8 point victories, he needs much bigger margins in the states he does win. Impossible? No. Improbable? Yes.

Wait, there’s more. We’re just talking about pledged delegates here. There are also unpledged delegates, better known as “superdelegates.” These are the party figures who automatically get to go to the convention and vote due to the innate poobah-ness of their position.

Lots of those superdelegates have already endorsed Clinton. A few have backed Sanders. Add those into the mix, and the former Secretary of State has 1,244 delegates, while the former mayor of Burlington, Vt., has 574.

Yikes! That’s a yyuuge deficit to make up a few delegates at a time. Sanders would need the equivalent of an average 40 percentage point victory in every state to accomplish that task.

In reality, the superdelegates are unlikely to sway the election by themselves. If primary season ends and Sanders has won a majority of pledged delegates in primaries and caucuses, past experience shows that the superdelegates will go along with the people’s decision.

But past experience in this case does not equate to future surety.

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“Sanders is such a party outsider, and there is such concern about how he’d do in a general election, that superdelegates might well try to swing the race toward Clinton after all. We just don’t know yet,” wrote Vox’s Andrew Prokup last week.

Here’s what this means for interpreting Tuesday’s primary results: On the Democratic side, net delegates won is by far the most important measure. Number of states won is not.