Some states have postponed their primaries over coronavirus concerns, and officials in Ohio, which is one of the four big states that was supposed to vote Tuesday, have suspended in-person voting.
But primaries go on in Florida, Illinois and Arizona. It's a crucial set of contests. Former Vice President Joe Biden has an approximately 150-delegate lead over Vermont Sen. Bernie Sanders, and 441 delegates are at stake Tuesday.
The candidates and their allies have spent nearly $18 million on ads in the three states voting Tuesday, with Sanders a little ahead of Biden. But Biden is favored to increase his delegate lead because the demographics of the states overall tend to favor him.
Here's a state-by-state guide on what to watch:
Delegates at stake: 219
Final polls close: 8 p.m. ET (most polls close at 7 p.m. ET, but the western part of the Panhandle is in the Central time zone).
This is the day's biggest prize, and Biden is the heavy favorite, leading in polls by 40 points.
Why? Biden has been winning older voters and black voters by wide margins, while Sanders has been winning younger voters. And Florida, on a whole, is a relatively older state. About 1 in 5 Florida residents is older than 65, higher than the national average of 16%.
In the 2016 Democratic primary, when Hillary Clinton beat Sanders there by more than 30 points, a quarter of voters were 65 and older, according to exit polls. Just 15% were younger than 30, lower than other states where Sanders has done well.
More than a quarter of Democratic voters were black, and while 20% were Latino — and Sanders has been doing well with Latinos — Florida's Hispanic population is different than the Hispanic population in the Southwest. In Florida there are fewer Latinos from Mexico and more from Puerto Rico and Cuba. Does Sanders maintain his margins with Latinos, given those differences?
Delegates at stake: 155
Polls close: 8 p.m. ET
Illinois has a significant chunk of delegates. In 2016, Clinton narrowly beat Sanders by less than 2 points.
Illinois also has a sizable share of black voters, and more than half the state's vote in 2016 came from Chicago's county. That year, 28% of the Democratic electorate was African American. Clinton won them by a huge margin. And Clinton won the urban areas of the state. Sanders, though, won white voters, especially white men — 64% to 35% — and he cleaned up in the more rural areas. Does Biden cut into those margins, like he did in Michigan?
Biden has been leading by a lot in polls, but this is the kind of state in which Sanders needs to make a stand to show he can change the narrative of this campaign.
Delegates at stake: 67
Polls close: 10 p.m. ET
Clinton won the Arizona primary by 18 points, and Biden is ahead in the polls by a wide margin, too.
There were no exit polls in the 2016 election in Arizona, and the wild card here is Latinos. While Arizona is 32% Hispanic, they made up just 15% of the 2016 electorate in the general election between President Trump and Clinton. The Arizona Democratic Party estimates that Latinos make up 36% of state Democrats, but whether they show up in those numbers in the primary could make the difference in whether Sanders has a chance here.
For places to watch, almost three-quarters of the vote comes from the counties that include Phoenix and Tucson. In most years, that makes campaigning in Arizona fairly easy.
But this year, with the novel coronavirus, both campaigns have canceled events. Even Sunday's debate, which was supposed to take place in Phoenix, was moved to a Washington TV studio with no audience out of caution for spreading the virus.
Turnout has been up in the Democratic presidential primaries, but how coronavirus fear affects turnout across these three states is going to be a big thing to watch.
NPR news assistant Elena Moore contributed to this report.