Voters appeared to be turning out in force Tuesday in the key presidential battleground state of Pennsylvania as Donald Trump and Joe Biden headlined a statewide election in which millions of ballots have already been cast.

Polls opened on a day when Pennsylvania recorded its highest single-day total of new coronavirus infections. The pandemic formed an Election Day backdrop that also included a police shooting and civil unrest in Philadelphia and the potential for a drawn-out legal fight over late-arriving mail-in ballots.

Election officials cautioned the winner might not be known for days as counties begin tabulating more than 2.5 million votes cast ahead of time.

Long lines formed at many polling places as voters also decided races for Congress, the General Assembly and a trio of statewide offices. State election officials reported few problems in the early going, though there were scattered reports that some polls failed to open on time because of glitches with equipment or late-arriving poll workers.

A line of about 150 people stretched a city block at one polling site in Philadelphia, where Shavere McLean, 36, a massage therapist, came bundled against the 39-degree chill. She also brought a chair, an apple, an orange and a cup of coffee.

“I tried to be prepared,” she said. McLean intended to vote for Biden, saying she is offended by Trump's behavior: “I just want a better leader, someone who cares about everyone.”

In Milford, a small northeastern Pennsylvania town close to the border with New York and New Jersey, most of the cars passing through the main intersection honked at Gail Just and her Trump-Pence sign. Just, 70, said she supports Trump because he "gets things done.”

Trump, the Republican incumbent who scored a surprise victory in Pennsylvania four years ago, and Biden, the Democratic challenger and native son, have frequently visited the battleground state, each seeing victory here as crucial to their chances of winning the White House. Biden visited his childhood home in Scranton on Tuesday before heading to Philadelphia.

Trump was relying on his supporters in small-town and rural Pennsylvania — the state’s Republican-dominated “T” — while Biden's hopes hinged on getting huge margins in the Democratic bastions of Pittsburgh and Philadelphia, as well as in the heavily populated, trending-blue Philadelphia suburbs. Polls leading up to Election Day showed a competitive race.

Trump has tried to sow doubt about the fairness of the election, saying the only way Democrats can win Pennsylvania is to cheat. Without evidence, he said late Monday that a court decision to allow Pennsylvania to count mailed ballots received up to three days after the election will allow “rampant and unchecked cheating” and will induce street violence.

State election officials have pushed back strongly, pledging a safe and secure election. Gov. Tom Wolf, a Democrat, promised accurate results, “even if that takes a little longer than normal." Democrats accused Trump of waging a campaign of voter intimidation and suppression.

For the most part, things seemed to be going fairly smoothly at the polls.

“We have not seen anything significant where it comes to voter intimidation or harassment. We are seeing enthusiastic partisan supporters in some places, but we are not seeing the kind of concerns that we may have had in the run-up to today,” said Suzanne Almeida, interim executive director of Common Cause Pennsylvania.

In the Philadelphia suburbs, Republicans and a local voter filed a lawsuit accusing Montgomery County officials of illegally processing mail-in ballots before Tuesday for the purpose of allowing voters to fix problems with their ballots. A county spokesperson, Kelly Cofrancisco, said “we believe our process is sound and permissible under the Election Code.”

The county-by-county tabulation is expected to last for several days because of a year-old state law that greatly expanded mail-in voting. The state Supreme Court, citing Postal Service delays, the huge number of people voting by mail because of COVID-19 and the strain on county boards of election, ordered counties to count mail-in ballots received up to three days after the vote, so long as they are mailed by Election Day.

The status of mailed ballots arriving after polls close at 8 p.m. has the potential to become significant if the nationwide result hinges on the outcome of Pennsylvania’s vote, and if the ballots are potentially decisive. Republicans have asked the U.S. Supreme Court to prevent any late-arriving ballots from being counted, citing state election law. The great majority of mail-in ballots have been cast by Democrats, according to state data.

Amid the legal wrangling, civil unrest in Philadelphia became a political issue in the closing days of the campaign.

The fatal police shooting last week of Walter Wallace Jr., a Black man holding a knife, sparked violent clashes between police and protesters, widespread vandalism and the ransacking of stores. Trump sought to use the unrest to push a law-and-order theme and castigate big-city Democrats as leading a “war against police.” Biden, meanwhile, expressed sympathy for Wallace’s family and called for “real police reform” while condemning the violence and looting.

Trump won Pennsylvania by less than 1 percentage point in 2016, eking out a surprise victory over Democrat Hillary Clinton to become the first Republican presidential candidate since 1988 to take the state. No Democrat has lost Pennsylvania but won the White House since Harry Truman in 1948.

The candidates visited the state at least once a week since the beginning of September, including a flurry of last-minute campaigning over the weekend and on Monday.

Though the presidential race sucked up most of the attention, there were other important races on the ballot.

All of Pennsylvania’s 18 congressional seats — currently occupied by nine Democrats and nine Republicans — are up for grabs.

A pair of Democratic incumbents, Attorney General Josh Shapiro and Treasurer Joe Torsella, are seeking reelection, while Pennsylvania will pick a new auditor general to replace term-limited Democrat Eugene DePasquale.

Control of the state House is also at stake, with Democrats needing nine seats to seize the majority from Republicans after a decade out of power. Democrats also have a gap to make up in the state Senate.

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Natalie Pompilio in Philadelphia and Associated Press writers Maryclaire Dale in Philadelphia and David Porter in Milford contributed.

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Find AP’s full election coverage at APNews.com/Election2020.