The coronavirus pandemic is presenting a singular test for American democracy, prompting states to postpone their primaries while already causing attorneys and voting-rights groups to take steps to ensure access to the November election in the event the outbreak is not contained by then.

Hardly any precedent exists for the dilemma now facing campaigns and voters in the states moving ahead with their contests, as Americans are warned they may need to “hunker down” to minimize casualties.

After the Centers for Disease Control and Prevention on Sunday advised against gatherings of more than 50 people, Sen. Bernie Sanders (I-Vt.) questioned whether elderly poll workers should be “sitting behind the desks, registering people,” while a deputy campaign manager for Joe Biden, the former vice president, had earlier encouraged voters who are healthy to “please vote.”

Of immediate concern is primary voting set to take place Tuesday in a handful of states — Arizona, Florida and Illinois — where election officials have taken steps they say will enhance voter safety. In Ohio, which had been scheduled to hold its primary Tuesday, Gov. Mike DeWine announced Monday that he would endorse a lawsuit asking a court to postpone in-person voting until June 2, saying the state could not both urge its residents to shelter inside and expect them to show up to polling places to vote.

But it remains unclear whether life will return to normal in time for primaries later in the spring, national nominating conventions this summer, and even the November election for president, Congress, and other state and local races. The uncertainty highlights the vexed relationship between partisan politics and public health, as Biden seeks to lock up the nomination and as Trump strives to hold on to power while responding to an escalating global emergency.

Congress controls the date of the general election, which has been inscribed in federal law since 1845. The Constitution requires the new Congress to be sworn in on Jan. 3, and it sets Jan. 20 as the beginning of the new president’s term.

Experts said Trump lacks the legal authority to change the date of the election. But some cautioned that increasingly stringent public health guidelines advising Americans to stay in their homes, or potential government-imposed lockdowns stretching into the fall, could present unprecedented obstacles to voting.

“If [Trump] calls out the National Guard and says people can’t leave their houses? That’s a crisis,” warned Richard L. Hasen, a professor of law and political science at the University of California at Irvine.

Some Democratic lawmakers moved this week, with the backing of voting-rights groups, to expand access to mail-in ballots and other remote voting options in time for November.

Sen. Ron Wyden (D-Ore.) planned to introduce a new version of a bill, joined by Sen. Amy Klobuchar (D-Minn.), that would require states to offer early in-person voting as well as no-excuse absentee and vote-by-mail options.

Their aim is to include the measure in a legislative package responding to the pandemic, though the issue already was exposing long-standing partisan disagreements over voting access.

A spokesman for Sen. Mitch McConnell (R-Ky.), the majority leader, declined to comment on the legislation. A Republican aide, speaking on the condition of anonymity to discuss legislative deliberations, threw cold water on the proposal, saying states were better left to decide on their own rules for voting.

Some of the largest states in the country are already looking ahead to November. Douglas A. Kellner, co-chair of the New York State Board of Elections, said Wyden’s bill is part of ongoing discussions among election officials, the governor’s office and legislative leaders. While there is concern about the potential for fraud in an all-mail election, he said, “the consensus is that we need to be evaluating options because there is a possibility that this could be a problem some months from now.”

History offers little guidance for today’s crisis.

Midterm elections unfolded in 1918 as the Spanish flu tore through the country. But political scientists and experts in election law reached back further to find a historical touchstone for the turmoil now engulfing a high-stakes presidential race.

In 1864, with the nation still rent by the Confederate rebellion, Abraham Lincoln insisted on standing for election, arguing, “If the rebellion could force us to forego or postpone a national election, it might fairly claim to have already conquered and ruined us.”

The primary calendar already has been disrupted.

Election officials in the states voting this week consolidated polling places — mostly moving them out of senior centers — and sought to absorb losses as elderly poll workers ran for the exits. Officials in Georgia and Louisiana postponed primary contests, and similar delays were possible in New York and Puerto Rico, among other states and territories. Elsewhere, state officials were dusting off statute books to determine for the first time how their emergency powers transformed their authority over the administration of elections.

About a dozen groups issued a statement on Monday urging election officials in Georgia, where the 2018 governor’s race prompted allegations by Democrats of voter suppression, to protect the right to vote as the state delayed its presidential primaries from March 24 to May 19. Coalitions of voting-rights organizations earlier sent letters to officials in states planning to press ahead with primaries on Tuesday asking them to extend voting hours and notify residents of changes in their polling locations, among other moves.

Some groups already advocating for alternative voting arrangements stepped up their efforts to expand mail-in options, with a focus on the general election.

The National Vote at Home Institute is set this week to release a plan for states and local jurisdictions to scale up options for at-home voting, said the group’s chief executive, Amber McReynolds, a former Colorado election official.

“What we need to do now is assume the worst case,” she said.

Vanita Gupta, the president and chief executive of the Leadership Conference on Civil and Human Rights, said the uncertainty about the duration of the outbreak makes it critical to begin mobilizing for November.

“States need to act in short order,” she said. “All of the policy fixes that they can enact — my hope is there will be measures put in place in the next week by states across the country.”

The American Civil Liberties Union is gearing up to conduct state-by-state advocacy urging elected leaders to expand mail-in voting and to conduct widespread public education about how to participate remotely, said Dale Ho, the director of the ACLU’s Voting Rights Project. He said litigation could play a role, too.

Meanwhile, a top Democratic lawyer is already looking to the courts to ensure voting is not compromised by the spread of the novel virus.

Marc Elias, who served as counsel to Hillary Clinton’s 2016 bid, said he and his associates are prepared to sue states to change their rules governing absentee and vote-by-mail opportunities, which are currently only afforded without an excuse in two-thirds of states. He is pursuing litigation in several states addressing rules for signature matching, postmark deadlines, and ballot assistance and collection — which could all be hurdles to broader mail-in voting.

“I can assure you that I and undoubtedly others will aggressively bring cases to ensure that every voter who is lawfully eligible to vote has an opportunity to do so and has their ballot counted,” Elias said.

Kory Langhofer, an attorney for Trump’s 2016 campaign and transition team, said the “unprecedented situation” made it difficult to predict how voters would react. He accused Elias, the Democratic attorney, of pursuing changes designed to “increase the rate at which his side’s likely voters participate.”

Democracy has never been impervious to disaster. Municipal elections in Florida were delayed in 2017 because of Hurricane Irma. Emergency legislation was necessary on Sept. 11, 2001, to postpone primary races in New York City when terrorists struck the World Trade Center.

But the grinding to a halt of public life presents a challenge unlike any other for the waging of a presidential race, said Juan Peñalosa, the executive director of the Florida Democratic Party.

“What are we telling voters?” he said. “We, like everyone, are following the guidelines, and I think voters should as well.”

Recommendations from the Centers for Disease Control and Prevention spell out ways for voters to “minimize direct contact with other people and reduce crowd size at polling stations.”

With that guidance in hand, campaigns and state parties are encouraging voters to participate in primaries unfolding on Tuesday. “It is okay,” Columbus City Attorney Zach Klein said Monday on a call with Biden supporters in Ohio. “It is safe.”

But going to the polls in person involves a new calculation of risks and benefits that voters must make, said Abigail Norris Turner, an epidemiologist at Ohio State University. “And that’s a calculation related to their own health and vulnerability and that of others in their household.”

Elise Viebeck and Cleve R. Wootson Jr. contributed to this report.