Voters turned 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 in the biggest test yet of the state's new mail-in voting law.

Even with so many votes already banked, lines were long 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, though there were scattered reports that some polls failed to open on time because of glitches with equipment or tardy 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 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 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 relied 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 as many as 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 fairly smooth 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.

Among other scattered problems, there were complaints about a heavily armed constable standing in the doorway of a polling place. The constable moved. “At first blush, yeah, it looks intimidating,” said Matt McDermott, Lycoming County's director of administration.

In Pittsburgh, a polling place couldn't open on time because the judge of elections' car was stolen, according to Allegheny County spokesperson Amie Downs. The car, since recovered, contained a suitcase with election paperwork and keys to a ballot scanner. At another Pittsburgh polling place, a poll worker was removed for causing a disturbance, Downs said.

And in Philadelphia, a poll worker improperly blocked a Trump campaign poll watcher from entering a voting precinct in the mistaken belief he wasn't allowed to be there, said Kevin Feeley, spokesperson for the Board of City Commissioners, which oversees elections. “Our judge of election dropped the ball. He made an honest mistake,” Feeley said. The poll watcher was eventually allowed in, he said.

Meanwhile, Republicans and a local voter outside Philadelphia filed a federal 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.

The state’s high court has not prohibited counties from allowing voters to fix their ballots, said Kelly Cofrancisco, a county spokesperson. “We believe in doing whatever we can to afford those who have legally requested and returned a ballot a fair opportunity to have their vote count,” Cofrancisco said.

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 as many as three days after the vote, so long as they were mailed by Election Day.

The U.S. Supreme Court rejected a Republican effort to block the counting of late-arriving mail-in votes, but it could revisit the issue.

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. The great majority of mail-in ballots have been cast by Democrats, according to state data.

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.

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