The former vice president was on course to expand his lead over Sen. Bernie Sanders, I-Vt., and cross the halfway mark toward the 1,991 delegates needed to clinch the nomination on the first ballot.
The primaries came as the coronavirus outbreak wreaked havoc on American life and sent the economy into a tailspin, although large numbers of votes had already been banked in the early voting period. Ohio postponed its primary at the last minute.
Below are some key takeaways.
Biden closer to becoming presumptive nominee
In Florida and Illinois, the two biggest prizes on the map Tuesday, Biden decisively won men and women, white voters and non-white voters, college graduates and non-college graduates, liberals and moderates, married and unmarried voters.
His lopsided margins suggest that many Democrats want the primary to be over. One of them is former Sen. Claire McCaskill of Missouri.
"The conversation is going to quickly turn to how and when does Bernie Sanders unite the Democratic Party," McCaskill said on MSNBC. "I think it is time. And Bernie's going to have plenty of delegates and power to influence the platform, because we all want to come together. So I do think the pressure is going to mount, especially at this time of crisis in this country, for the Democrats to unite behind clearly the voters' preference."
David Plouffe, Barack Obama's campaign manager in 2008, was more categorical: "Joe Biden is the Democratic nominee. The general election is set."
Biden still struggling with young voters
One positive sign for Sanders was that he continued to win voters under 45 years old by large margins in all three states Tuesday. Sanders won those by 13 points in Florida, 37 points in Illinois and 52 points in Arizona.
Sanders needed young progressives to turn out in big numbers to outvote older moderates — but voters under 30 fell slightly from 2016 levels as a share of the electorate in Illinois and Florida.
Biden acknowledged his rival's strength with young people.
"Let me say, especially to the young voters who have been inspired by Senator Sanders: I hear you. I know what is at stake. And I know what we have to do," he said. "Our goal as a campaign, and my goal as a candidate for president, is to unify our party — and to unify our nation."
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Sanders on the brink
One way to know Sanders is in trouble? When he's losing "very liberal" voters in major states, as he did in Florida with 44 percent to Biden's 48 percent, according to NBC News primary polls.
He lost "liberal" voters in Illinois by 44 percent to Biden's 51 percent. The defeat in Illinois was particularly disappointing; he came within 2 points of victory there in 2016.
Sanders' hopes of turning things around now would hinge on his delivering massive wins in big coming states, although it's not clear where he could do that.
The senator addressed the country on the coronavirus crisis in livestreamed remarks before polls closed. His campaign said he didn't plan to speak about the results Tuesday night. There was no indication that he'd leave the race, as many allies want him to stay in and use his leverage to nudge Biden toward more progressive policy positions, which he has already been successful at doing.
Elizabeth Warren dropped out soon after Super Tuesday, but her non-endorsement continues to loom over the primaries. An endorsement of Sanders, whom she was ideologically aligned with, might have given him a fighting chance with college-educated white women, who were a core constituency for the senator from Massachusetts before she ended her campaign.
Sanders got routed among white women with college degrees, losing them to Biden by 39 points in Florida, by 20 points in Illinois and by 9 points in Arizona, according to NBC News primary polls.
Still, given Biden's margins, it's doubtful a Warren endorsement would have been enough for Sanders.
Florida turnout high despite coronavirus
Democratic turnout in Florida was projected by NBC News to top 2 million, eclipsing the 2016 total of 1.7 million, attributable in large part to early voting and mail-in ballots.