The former vice president now has a nearly 150-delegate lead over the Vermont senator after winning four of the six states that voted Tuesday, according to NBC News. Nationally, Biden has 837 delegates to Sanders' 689, as of 8 a.m. ET.
Biden appears on track to win Mississippi and Missouri by lopsided margins, which will lead to big delegate hauls since Democrats award delegates in proportion to the margin by which candidates win in each state. He won Idaho by a narrower margin, NBC News projected early Wednesday.
Biden has also won the biggest state on the map, Michigan, with 125 delegates at stake. Sanders had won the state at a crucial moment in his 2016 battle against Hillary Clinton and he staked his 2020 comeback on it.
However, NBC News projected Wednesday morning that Sanders has won North Dakota, which has 14 delegates up for grabs, while Washington, the second-biggest state on the map with 89 delegates, was still too close to call.
Sanders would likely need to win each state by massive margins to make up for the delegates Biden is now projected to gain.
Biden came into this round of contests with a 77-delegate lead over Sanders after his strong showing last week on Super Tuesday.
Sanders could still pick up more delegates from those Super Tuesday states, some of which are still counting ballots, since the largest caches of yet-to-be-allocated delegates are in states the Vermont senator is leading: California, which NBC News has still not called and with 63 delegates yet to be assigned, Utah, with 20 delegates, and Colorado, where 16 delegates are on the line.
And there is no obvious break in the clouds on the horizon for Sanders.
Next Tuesday will be the second-biggest day of the entire primary in terms of delegates, following only Super Tuesday, when Florida, Arizona, Illinois and Ohio vote. Biden has a wide lead in the few public polls that have been taken of those states, with a recent Florida Atlantic University poll showing Biden ahead 61 percent to 25 percent.
A week later will come Georgia, the type of Southern state with a large black population where Biden has been especially strong this year. Sanders could hope for success in Puerto Rico later that week given his strength with Latinos.
April 4th is the next best slate of states for Sanders, with contests in Alaska, Hawaii, Wyoming, all of which Sanders won in 2016. But they together have roughly the same number of delegates as Louisiana, which also votes that day, where a larger African-American population gave Hillary Clinton a big win four years ago.