By STEVE PEOPLES AP National Political Writer

Voters are being asked to navigate curfews, health concerns and a sharp increase in mail balloting on Tuesday as elections take place from Maryland to Montana. 

Four states were originally scheduled to vote in April but delayed their contests because of the coronavirus outbreak. Pennsylvania offers the day's biggest trove of delegates and represents a high-profile test case for Republicans and Democrats working to strengthen their operations in one of the most important general election battlegrounds.

"We think we're prepared," Pennsylvania Democratic Party Chairwoman Nancy Patton Mills said. "Thank goodness we have the opportunity of working this out in the primary because we don't know where we'll be with the pandemic in November."

Joe Biden needs to win 89% of all delegates at stake on Tuesday to formally clinch the nomination, but his role as his party's clear presidential nominee is not in danger should he fall short. With a dominant showing on Super Tuesday in early March, the former vice president pushed out all his major opponents. He will almost certainly secure the needed delegates later in the month if necessary.

Still, Tuesday offers a historic opportunity for the 77-year-old Democrat, who is waging his third presidential campaign and who hopes to amass as many delegates as possible to show strength before going up against President Donald Trump. 

Vermont Sen. Bernie Sanders is not actively campaigning, having suspended his operation and endorsed Biden, but his name will appear on the ballots. On the eve of Tuesday's primaries, senior adviser Jeff Weaver encouraged progressives to vote for Sanders anyway.

"People who support Bernie Sanders and his agenda, who want to maximize the influence of progressives at the convention, should cast their vote for Bernie Sanders," Weaver said, reminding voters that the Vermont senator is seeking leverage to shape the party's platform and rules. 

The comments serve as a reminder that Biden may have no legitimate Democratic rivals remaining but must still win over skeptical activists from his party's far-left flank, who worry he's too close to the political establishment. 

Party unity will likely be an afterthought this week, however, as more immediate health and safety concerns dominate the national conversation.

The coronavirus death toll has surged past 100,000 nationwide, and thousands of new cases are reported each day. At the same time, several major cities, including some voting Tuesday, are grappling with protests following the killing of George Floyd, a handcuffed black man who died after a white Minneapolis police officer pressed his knee into Floyd's neck for several minutes.

Washington, D.C., Mayor Muriel Bowser ordered a 7 p.m. curfew, though voting places will be open until 8 p.m. Voting has been deemed essential, and city officials say voters will not be subject to arrest if they cast ballots during the curfew. It's much the same in Philadelphia, where officials have promised that voters would not be arrested should their city's 6 p.m. curfew be extended for a fourth consecutive night. 

"We are in unique times, and voting is a unique challenge for people," said Josh Schwerin, chief strategist for the pro-Democrat super PAC Priorities USA. He said that his organization and others will be watching closely on Tuesday "to see how well it works, where issues are and where obstacles have been put in place."

Political groups have had to adjust as some states move to a system that relies largely on voting by mail. They include Montana, where all 56 counties decided to vote entirely by mail. Voting rights watchdogs in multiple states have expressed concerns about access to mail ballots, confusion about deadlines and a shortage of poll workers that could lead to long lines. 

Those voting Tuesday include the District of Columbia, Idaho, Indiana, Maryland, Montana, New Mexico, Pennsylvania, Rhode Island and South Dakota. Another state holding primary elections, Iowa, chose its presidential nominee early in the year and focused on other offices.