State leaders, voting experts and advocates said they were preparing for an unusual level of confusion and chaos today as voters cast their ballots in a historically bitter presidential race.
Early voters in some states faced hours-long lines in the days leading up to today. Democrats have filed a flurry of last-minute lawsuits alleging voter intimidation by Donald Trump supporters. And there have been some heated polling site confrontations between Trump voters and Hillary Clinton backers.
Election monitors were especially worried about the spectre of voter intimidation after calls by the Republican candidate for his supporters to stake out polling places and watch for fraud.
Election officials in Pennsylvania and Arizona were so concerned in recent days about potential intimidation that they issued advisories spelling out what types of threatening behaviour are banned and the exact dimensions of buffer zones surrounding polling places.
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"Individuals who conspire to interfere with a person's right to vote can face up to 10 years in prison," read a notice issued by Pennsylvania's Secretary of State after Trump singled out Philadelphia as a place for his supporters to scrutinise.
The concerns have led to extensive contingency plans and a heavy workload for state officials, lawyers and election experts, who have been trying to monitor voting problems and troubleshoot them in real time.
"The number and tenor of calls we've received about problems, the amount of litigation you already see, it all reflects the moment we're in," said Kristen Clarke, president of the nonpartisan Lawyers' Committee for Civil Rights Under Law.
Today, Clarke's group will be running the nation's largest independent effort to field voter complaints, questions and problems. The nonpartisan umbrella organisation called Election Protection will have 4500 legal volunteers answering calls to its hotline, and 2500 volunteers stationed at polling sites around the country.
Clarke's group has set up a war room at its headquarters in Washington - a cavernous space where hundreds of pro-bono lawyers are stationed at computers as they answer voters' calls.
But the intensity of this year's election, Clarke said, had exhausted many of her lawyers and volunteers before Election Day even arrived. These attorneys have fought attempts to purge residents from voter rolls in New York, Georgia and North Carolina. They've investigated reports such as one last week during early voting in Cleveland that a security guard was turning away voters because a parking lot with available spaces was supposedly full.
Among Clarke's biggest concerns for today were long lines.
On Monday, the line at the sole early voting station in Cincinnati stretched at least 400m, with estimates of those waiting in the thousands. Food trucks at one point handed out free tacos and cupcakes as encouragement. Early voters also faced long lines in California, Arizona and Nevada.
In addition to those states, election experts say, extensive waits are a particular concern in Florida and Ohio, given the states' election history. In 2012, some precincts in Miami were so clogged that some voters were still casting ballots at 1am as Barack Obama was making his victory speech.
In the most hotly contested states, many lawyers have been deployed by both parties as well as independent voting rights groups. The lawyers have been stationed inside polling places to help resolve disputes.
The Justice Department sent more than 500 workers to 28 states to monitor polls on Election Day. The deployment is significantly smaller - by at least 280 from 2012 - because of a Supreme Court decision that Justice officials think limits the federal Government's role at polling places.
With just five lawyers on its voting rights team, the ACLU had to draft its Election Day strategy more judiciously - deciding to send them to Philadelphia, Cleveland, Milwaukee, Miami and the last location most likely North Carolina.
"The state laws vary so much that you have to prepare for it. And the states where it's close, that's where the temptation will be greatest for voter intimidation tactics," said Dale Ho, the ACLU's voting rights director.
His lawyers are landing with restraining orders and injunctions pre-drafted and ready to be filed, he said. "It's like those fire tools behind the glass: 'Break in case of emergency'."
It remains unclear how widespread the much feared threat of voter intimidation is, or whether it will have materialised.
A group called Stop the Steal - run by longtime Republican operative and outside Trump adviser Roger Stone - recruited "vote protectors" to stand outside polling stations. As of yesterday, the group had signed up 3191 people pledging to do so.
"This is our best chance to stop the Democrats from stealing the election from Donald Trump. Help us fight a rigged system," the group said on its website.
Democrats have filed several lawsuits against the Republicans, and Stone's group accused them of planning voter intimidation. In an email last week, Stone called the lawsuit "bogus" and said that Stop the Steal "is conducting a neutral, scientifically based exit poll" that would only involve interviews after people cast ballots.
The cases have added to the unusually heavy flood of last-minute legal battles over issues such as voters being purged from rolls, registration deadlines and election access.
During early voting in Florida, Trump supporters were videotaped using bullhorns to shout at Clinton supporters and voters outside a polling site in West Palm Beach. Video posted online showed a Trump supporter yelling, "How many Syrian refugees, Muslim refugees, are you taking into your home?"
Voting advocates complained to authorities, who then ordered the individuals to move farther away from the polling site.
Election officials in several states said the potential for clashes had caused them to step up coordination with police ahead of today's vote. And many state authorities were already on alert after federal officials warned of a vague possible threat of a pre-election attack by al-Qaeda as well as the possibility of Russian hackers trying to sow disinformation.
Some were worried that more mundane problems could wreak havoc. New voting restrictions - including some laws requiring photo ID - are in place for the first time in more than a dozen states, sowing potential for confusion.
And legal fights over the laws have complicated the matter further. In Texas for example, after a federal court ruled that the state's voter ID law discriminated against minorities, fliers remained up at some polling places, warning voters that photo ID was required.
With all there is to worry about, voting advocates had one more concern to add to the pile: That some voters would be discouraged enough to stay home.
"I'm worried about the worrying," said Wendy Weiser, an election expert at the nonpartisan Brennan Centre.
"There's so much anxiety across the country. You have many people worried for the first time about the integrity of our election system, which is actually pretty good. You have people talking about needing to watch other people as they watch voters. It's crazy."
B-2 bombers and refuelling jets flew off the California coast overnight.