The victories mean Biden will extend his delegate lead over Sen. Bernie Sanders, I-Vt., one week after initially gaining it on Super Tuesday, even as three states voting Tuesday have yet to be called: Idaho, North Dakota and Washington state.
Here are some takeaways.
1. The delegate gap widens
We don't yet know the full results, but we can say with confidence that Biden's delegate lead will grow substantially once we get them all. That's because of both the trio of early wins - in states making up 63% of the delegates available Tuesday - and the margins of victory.
Michigan and Missouri were very close between Sanders and Hillary Clinton in 2016, but the margins Tuesday were so big that Biden was called the winner in both shortly after polls closed. There was also an outside shot that Sanders would fail to clear the 15% threshold for delegates on Mississippi, which would be yet another big setback for him, despite the state's relatively small number of delegates (36).
When you throw all of that on top of the fact that the slow-counting California primary hasn't produced the kind of delegate comeback Sanders was hoping for in the days after Super Tuesday, you start to see a yawning gap in the delegate race. All that remains to be seen now is how much the margin increased.
2. Michigan giveth, and Michigan taketh away
The Great Lakes State was Sanders' ticket to the party in 2016, with his shocking upset there - after polls showed him down double digits - serving notice after Super Tuesday that Democratic voters weren't quite ready to anoint Clinton. On Tuesday, it may have served the opposite function.
One of the more interesting findings in the early exit polls was this: Among voters who decided on their candidate before this month, Sanders took a majority of voters, compared with Biden's 4 in 10. But about half of voters said they decided this month - after Biden began his comeback with a big win in South Carolina - and they went for Biden more than 2 to 1.
In other words, Sanders was arguably primed to win the Michigan primary just 11 days ago, and then things changed significantly.
It's an especially big win for Biden because he did it statewide and with surging turnout. While 1.2 million people voted in the state in the 2016 Democratic primary, that number was estimated at 1.7 million on Tuesday. Part of that could have been because there was no competitive Republican primary. In a state that Democrats badly need to win in the general election after losing it narrowly in 2016, expect Biden to make that part of his pitch: I can get voters to the polls in Michigan, because I just did it.
3. Sanders hits his demographic wall again
It was incumbent upon Sanders at the start of this race to build on his 2016 coalition - or hope for the crowded race to last longer. But now that the field is down to two candidates, two familiar and potentially impassable problems remain for Sanders.
The first is his performance among black voters. His inability to win them over in 2016 was pretty much insurmountable in his race against Clinton. And now that it's just him and Biden, the story is virtually the same. In Mississippi, he took 11% of the black vote in 2016 and was taking 13% on Tuesday according to exit polls. In Missouri, he took 32% in 2016 and was taking 28% on Tuesday. In Michigan, he moved from 28% four years ago to 29% Tuesday.
The other is his reliance upon younger voters. Sanders' campaign has argued that he can win in the general election because of his unusual appeal to this low-turnout group, but that hasn't panned out in the earliest contests. And it deserted him again Tuesday. Voters between 18 and 44 were 40% of the vote in Mississippi in 2016, but just 31% on Tuesday. In Missouri, there were 41% in 2016 and 33% on Tuesday. In Michigan, youth turnout was the reason he pulled a shocking upset in 2016, but 18- to 44-year-olds' share of the vote dropped from 45% then to 38 percent Tuesday.
Sanders needed to expand his appeal to older people and to working-class white voters, but he's regressed on both of those too.
4. The schedule is starting to favor Biden
As ominous for Sanders as what happened on Tuesday night was this: It might have been his best shot at actually getting back in the race this month.
In 2016, Sanders actually carried four of the six states that were up Tuesday - the three yet-to-be-determined states and Michigan. He also came within 0.25% in Missouri. The opportunity was there, but he lost ground.
Next week, the primaries will be Arizona, Florida, Illinois and Ohio. Sanders lost all of them in 2016, including three of the four by double digits. All of them are demographically tough for him, and all of them are relatively big delegate prizes, with about 200 more delegates at stake than there were on Tuesday night.
The week after that comes Georgia, a state with a large black population and many delegates.
In other words, it becomes difficult to see how Biden does not exit this month with a large delegate share. Sanders needs to do something to change the race in a big way, but it's not clear that he even has the opportunity over the next two weeks.
Also consider this: The only states Sanders has won east of the Mississippi River are his home state of Vermont and the neighboring state of New Hampshire, which was very close. There are not enough remaining delegates in the western part of the country for Sanders, especially with California out of the way.
5. Biden's electability case grows
It wasn't just the surge in turnout in Michigan. Biden's wins also spanned the Democratic electorate in one very significant way.
The following is true in all three states that Biden won early Tuesday night: He won nonwhites who both went to college and didn't - no surprise there - but he also won whites who went to college and those who didn't. The closest he came to losing any of them was in Michigan, where non-college whites were somewhat close.
You can't directly correlate primary results to the general election, but for a Democratic Party that is twitchy about putting forward an electable candidate, you could do a lot worse than showing that kind of appeal to all corners of the party.
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