Democratic presidential candidate Bernie Sanders. (Michael Cali/AP)

Six days ago, Bernie Sanders pulled off one of the great upsets in modern politics — surging from more than 20 points behind in the polls to edge out Hillary Clinton and win Michigan’s Democratic presidential primary.

It was remarkable! It was historic! And it netted him four more delegates than Clinton in the state. Meanwhile, in Mississippi, Clinton won with more than 80 percent of the vote — and gained 28 more delegates than Sanders.

On the best night of the Sanders campaign to date, he fell 24 more delegates behind Clinton in the race for the Democratic nomination.

That increasingly challenging math is what Sanders must confront Tuesday as voters in several large states — including Florida, Illinois and Ohio — go to the polls.

Bernie Sanders won Michigan on March 8 by getting votes from several key groups. (Peter Stevenson/The Washington Post)

As of today, Clinton has 1,231 delegates to Sanders’s 576 — a lead of 655. That means that Clinton has 51.7 percent of the 2,383 delegates she needs to become the Democratic Party’s nominee.

Subtract superdelegates — Clinton is dominating even among this group of elected officials and party luminaries — and she has 766 delegates to Sanders’s 551, a margin of 215. (Worth noting: That is a wider lead than the margin by which Clinton ever trailed then-Sen. Barack Obama in the long slog of the 2008 primary race.)

That lead may not seem momentous. After all, almost 3,000 delegates are yet to be allocated in the primaries and caucuses to come. The problem for Sanders is that Democrats allocate their delegates proportionally in every state — meaning that between now and when the process ends June 7, there is no state where Clinton will be shut out.

Winning, then, is not enough for Sanders. He has to win by a lot to make up any real ground.

Clinton has already done that. Take, for example, Alabama. She won there March 1 by 59 points and gained 38 more delegates than Sanders. Or Georgia on that same day, beating Sanders by 43 points and netting 55 delegates. Or the aforementioned Mississippi, where Clinton’s 66-point win translated to a net gain of 28 delegates.

Sanders’s one big win came in New Hampshire’s primary. But his 22-point margin translated to a net delegate gain of zero because six superdelegates pledged to Clinton, bringing her delegate gain up to match his. Similarly, in the Colorado caucuses, Sanders won by 19 points but the superdelegate math meant the candidates each took 38 delegates.

Look at the next set of big contests, to be held Tuesday. Four states have more than 100 delegates to give out: Florida (246), Illinois (182), Ohio (160) and North Carolina (121).

Polling released Sunday morning suggests that Sanders has a big hill to climb. Clinton leads the senator 61 percent to 34 percent in Florida and has an edge of 58 percent to 38 percent in Ohio, according to NBC- Marist surveys. The race in Illinois, according to NBC-Marist, is closer, with Clinton at 51 percent and Sanders at 45 percent.

But wait, you say. Polling in Michigan had Sanders down 20 points and he won there. So this polling could be wrong, too.

Sure. It could. The problem for Sanders is this: Let’s say each of the NBC-Marist surveys is off by 20 points in Clinton’s favor. (Note that this is a thought experiment. I very much doubt a credible pollster such as this one would be off by even close to that amount.) That would mean Sanders loses Florida by single digits, essentially ties Clinton in Ohio and wins Illinois by 15 points. The delegate allocation from that trio of results? It would almost certainly favor Clinton.

Sanders is in a position where winning states is not close to enough if he wants to be the party’s nominee. He needs to start winning big states by big margins. As in winning Illinois or Florida by 30 or 40 points.

That seems very unlikely either Tuesday or beyond. If past votes are any guide, it will be a tough road for Sanders. There have been seven election nights in the race so far; Clinton has netted delegates in six of them, while the two candidates fought to a draw in the seventh (New Hampshire).

None of this means that Sanders can’t — and won’t — keep running. Winning states matters in terms of perception and keeps the wolves from his door. But winning states and emboldening your supporters aren’t the same as taking concrete steps to reduce or eliminate Clinton’s delegate lead.

That looks to be a near-impossible task for Sanders unless the numbers in the states to come start changing quickly.