As campaign season heads into its fraught, final days, police departments across the country are bracing for Election Day, mobilizing officers as they prepare for the possibility of voter intimidation, unrest or violence. 

These preparations include extensive planning exercises, efforts to monitor early-voting sites and concerns about how long any unrest could last if the election results are not immediately known or challenged in court.

While law enforcement agencies routinely plan for elections, officials say this year's preparations are unusually extensive because of the sheer levels of anxiety and toxicity across the country - with fears that a modern American election could give way to potential violence.

"I don't think we've seen anything like this in modern times," said Andrew Walsh, a deputy chief with the Las Vegas Metropolitan Police Department.

Tens of millions of Americans have cast ballots at early-voting sites, and tensions and accusations of voter intimidation have already flared up during this process.

Many Americans still plan to turn out on Election Day, so law enforcement officials have conducted drills involving multiple agencies to try to game out what could happen. Police officials in multiple cities stressed that they have no information about specific threats relating to the election, but are instead preparing to make sure they are ready.

"When you look at previous elections, there's always been the concern when you have large crowds ... we know [that] can be a target for someone who has an agenda," Walsh, who runs the Las Vegas police's homeland security division, said in an interview. The difference, he said, is the "magnitude" of the election and the weight people are attaching to it.

Walsh said law enforcement officials have to consider three distinct periods around the election. First is early voting period, which offers an extended stretch of public gatherings and possible friction, followed by Election Day itself, which could offer much the same in a concentrated single period.

Then comes the third, most uncertain period: however long it might take for an outcome to be declared. That window, experts say, could offer the greatest risk for potential danger.

"We just don't know how long this is going to take, or what this is going to look like, once this is over ... and no matter who wins, somebody's not going to be happy," Walsh said.

David Brown, the Chicago police superintendent, said at a news conference that city officials have held three tabletop exercises and plan at least one more "so we can have the best response regardless of whatever scenario happens on Election Day."

He said many other cities are doing the same.

"We are all in conversations with our counterparts across the country about what we might expect. But everything is uncertain. And so we're trying as best we can to anticipate any hazard that might happen," he said.

Those potential hazards, Brown said, range from bad weather to large protests to gatherings that could include "embedded agitators that might loot or cause violence or destroy property."

In New York, authorities conducted a tabletop exercise in recent weeks looking at the time leading up to Election Day and the following day, according to John Miller, the NYPD's deputy commissioner of intelligence and counterterrorism.

The exercise included "natural disasters, protests, violence, suspicious packages, explosive devices" among the litany of scenarios the department contemplated. He emphasized at a briefing this week that there is no credible threat to New York City or Election Day.

President Donald Trump repeatedly has attacked the integrity of the election, making false claims about voter fraud and urging his supporters to monitor activity at the polls, while his administration has warned about the danger posed by extremists who could target election-related events.

The Department of Homeland Security said in a threat assessment released earlier this month that domestic violent extremists could target events connected with the presidential campaigns, the election or the post-election period. It wrote that "campaign-associated mass gatherings, polling places, and voter registration events, would be the most likely flash points for potential violence."

As part of their preparations to protect such gatherings, police have begun shifting personnel to ensure larger numbers of officers are deployed or available.

Police in Madison, Wis., are planning on having significantly more staffing for this election than previous ones, and they are also undergoing greater preparations, Victor Wahl, the acting police chief, said in an interview.

In prior years, Wahl said, officers might get reminders before elections about things like electioneering and polling places, just because that was not something they dealt with every day.

"Certainly this year, what we've gone through the last few months, the national climate and things like that, we're spending a lot more time on it," Wahl said.

Police across the country have limited time off around Election Day to keep officers around. In Dallas, all sworn personnel must wear uniforms in case they need to be deployed, a spokesman said.

Officials in Phoenix and Portland, Ore., said they would increase staffing, while the Los Angeles Police Department also said they were prepared to deploy more officers as they anticipate possible protests. Some departments said they were ready to shift deployments as needed by circumstances on the ground, while others declined to detail their staffing or operational plans.

D.C. Police Chief Peter Newsham told local lawmakers that within law enforcement, "it is widely believed that there will be civil unrest after the November election regardless of who wins," as well as during the inauguration in January.

Preparations are also extending to polling locations and early voting. Police in Charlotte said they would check on polling areas there, while the Miami-Dade police - who noted they are only allowed in polling locations to vote or if allowed by precinct clerks - said they would add early-voting sites to patrol routes to keep an eye for any issues.

In New York, where early voting begins Saturday, Terence Monahan, chief of department for the NYPD, said at a news conference that his force would have "a presence everywhere people come to vote" to make sure everyone can safely cast their ballot.

When asked if police could remain impartial there after the largest New York City police union endorsed Trump, he said his officers would take no side while on the job.

"When we put on this uniform, we are apolitical," Monahan said. "We have no stance one way or the other."

Trump also sparked criticism with his call over the summer to have law enforcement officials patrol voting sites, a tactic that historically was used to scare voters of color.

"There are many folks who are rightly triggered and intimidated by just the sight of armed police in and around the polls," said Craig B. Futterman, a law professor at the University of Chicago and director of the Civil Rights and Police Accountability Project.

Police and law enforcement officials often spent recent elections trying to stay away from polling sites specifically to avoid giving the impression of any voter intimidation, said Gregory Shaffer, a former FBI agent.

This year, Shaffer said, he thinks police will likely need to be more focused on the streets, particularly given the potential for clashes between opposing groups. Police will likely be on standby, he said, and may do things including pre-staging riot police on buses near possible protest sites.

The most worrisome time frame heading into November is likely not the election itself but the post-election period, said Irfan Nooruddin, a Georgetown University professor and an expert in democratization and international politics.

When violence happens in other countries around an election, he said, it is usually after the vote, "in which the losing side, or the side that expects to lose, doubles down using violence to delegitimize the process."

Shaffer, the ex-FBI agent, said this year is the first time "that police are really, I think, in fear of what may happen on Election Day." Shaffer also said he believes police officers hope the election is not drawn out or close, regardless of who wins.

"They're all hoping for a landslide for one of the candidates and so a decision can be made quickly, so the country can move forward and their city can move forward," he said.

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The Washington Post's Peter Hermann contributed to this report.