A round of defeats for Bernie Sanders in Tuesday’s primary contests has put the Vermont senator in a deeper delegate hole — and his scenario for a comeback against Joe Biden is growing more implausible.

“Last night, obviously, was not a good night for our campaign from a delegate point of view,” Sanders said at a press conference in Burlington, Vermont, on Wednesday.

It’s true. Biden didn’t just win the most states — he won several landslides. Biden has expanded his lead over Sanders from 97 delegates to 162. Though delegate allocations are far from final, the Associated Press estimates the former vice president will get +29 in Mississippi, +20 in Michigan, and +17 in Missouri. Our partners at Decision Desk have a slightly more conservative overall estimate but predict a similar net advantage for Biden.

Sanders did score a victory by taking home the North Dakota caucuses, but the outcome was narrow enough and the state is small enough that he’ll just get a net delegate advantage of +2. Biden, though, won Idaho’s primary, and got his own net delegate advantage of +2 there — canceling out Sanders’s tiny gain.

The final state that hasn’t yet been called is Washington, which uses a vote-by-mail system and is slower to count votes. In Washington’s current count, Sanders is barely leading Biden, which would mean a delegate draw. But some analysts believe that as more late-arriving mail ballots are counted, Biden has a chance to pull ahead and potentially expand his delegate lead even further.

The big picture is that Sanders needed to make a national comeback after Super Tuesday. If he had done so on Tuesday, he would have needed to change the national dynamics of the race and sharply improve his performance. But instead, the opposite happened. Democratic voters backed Biden even more strongly.

And while it remains theoretically possible that Sanders can catch up, the window for him to do that is rapidly closing — and it could shut for good next week.

How to understand the Biden-Sanders delegate count

Though Biden won 10 out of 14 states on Super Tuesday, the delegate lead he emerged with wasn’t insurmountable. About 62 percent of pledged delegates remained to be allotted, so it would be possible for Sanders to make a comeback, if he started winning.

But Sanders would have to do it fast. Though voting in Democratic primaries continues through June, the calendar is frontloaded, with contests piled on both Super Tuesday and the two weeks afterward. After this Tuesday’s primary, just 53 percent of delegates remain. After next Tuesday, March 17, just 38.5 percent will be left.

Now, Democrats’ proportional delegate allocation rules will make it difficult for Biden to actually clinch the nomination (getting the 1,991 pledged delegates necessary for a majority) for some time.

However, those same rules make it difficult for a candidate who falls significantly behind in delegates to catch up. Narrow wins won’t cut it, since they amount to basically a draw in the delegate count (each candidate gets about half the delegates at stake). Landslide victories are what really matter.

Overall, you can think of the math in this way:

Before Tuesday, Sanders needed to win about 54 percent of delegates remaining to pass Biden in the pledged delegate tally.

However, on Tuesday, Sanders only won about 40 percent of delegates (per the AP’s current count).

That poor performance means the bar has been raised for Sanders going forward. He’d now need more than 57 percent of remaining delegates to end up with more than Biden.

In theory, it’s quite possible for one candidate to win 57 percent of the delegates in a two-way race. But that candidate would have to start winning solid or landslide victories in most states.

The problem for Sanders is that he has instead been losing in most states — and that he is rapidly running out of time to turn this around.

March 17 could be the knockout blow

Sanders’s prospects look particularly grim because in just six days, on March 17, there’s a massive delegate haul up for grabs, as Florida, Illinois, Ohio, and Arizona will all hold primaries.

Biden is expected to win Florida overwhelmingly, and the Michigan result may suggest he’s strongly favored in Illinois and Ohio too. Sanders hopes that Hispanic voters in Arizona can power him to victory there, but there are also many older white voters in Arizona who will likely back Biden.

Overall, though, March 17 is the day where the delegate math for a Sanders comeback can change from implausible to near-impossible.

If Biden wins 60 percent of delegates on that day (about the same percentage he won this Tuesday), Sanders would then need to win 66 percent of all remaining delegates to catch up.

And given voting results and demographic patterns of support so far, that’s just incredibly unlikely to happen — unless there’s some sort of seismic transformation in the race that decimates Biden’s support.