Technically, it’s not too late for Mr. Sanders to mount a comeback. Heading into tonight, he trails Mr. Biden by only around five percentage points in the pledged delegate count, according to our estimates. With 62 percent of delegates yet to be awarded, he would need to beat Mr. Biden by only three points the rest of the way to retake the delegate lead. On paper, it’s not a daunting deficit.

The problem for Mr. Sanders is that it’s a daunting deficit when you add in post-Super Tuesday national polls showing Mr. Biden ahead by around 20 percentage points. The polls are consistent with the Super Tuesday results, which showed Mr. Biden winning by a wide margin among voters who cast ballots after the South Carolina primary. It would take a big change in the race for Mr. Sanders to beat Mr. Biden by three points the rest of the way.

Mr. Sanders has few natural opportunities to fundamentally change the race, and realistically he has only until next Tuesday. Before then, there’s the next debate, on Sunday, and the contests tonight. In 2016, he carried four of the six states voting today. That includes Washington and Idaho — which tend to have liberal Democratic electorates — and North Dakota. These would seem to be some of the very best states that remain for him. And then there’s Michigan, the state where he posted the signature win of his 2016 bid.

But these states are not as favorable to Mr. Sanders as they were then. Washington and Idaho are no longer caucuses, a format that tends to favor him. North Dakota is now a firehouse caucus, which is essentially a primary run by the Democratic Party, rather than by the state, but with fewer polling places than usual. So Mr. Sanders may not have the same caucus edge that he had in 2016 there either.