Nuns in Erie falsely accused of perpetrating voter fraud. A suburban Philadelphia election-line snafu amplified worldwide in under half an hour. A Tuesday night rallying cry by Donald Trump: “They have already started cheating in Lancaster.”

Across Pennsylvania, local and state officials are warning that efforts by Trump and his supporters to call into question the integrity of the presidential election in the crucial swing state are ramping up—before a single ballot has been counted.

The state’s top election official, Republican Secretary of the Commonwealth Al Schmidt, said Wednesday that in just the previous 24 hours, several widely disseminated videos lacked context or were false.

“Sharing social posts filled with half-truths or even outright lies is harmful to our representative democracy,” he said at a press conference, urging voters to get information from trusted sources and adding that “mis- and disinformation” about the state’s election is “likely to continue in the coming days and weeks ahead.”

Authorities such as Schmidt have said that they worry the attacks could erode faith in the system while many citizens are still preparing to vote, and that it is setting the stage for fierce post-Nov. 5 legal challenges.

“I don’t think it’s a secret how important Pennsylvania is in determining who will win the race for the White House or control of the Senate,” Schmidt said, of the forces drawing election skeptics to the state.

Pennsylvania Secretary of the Commonwealth Al Schmidt and Philadelphia City Commissioners Chairman Omar Sabir and Seth Bluestein host a walkthrough of the ballot counting center in Philadelphia, Pennsylvania, U.S., October 25, 2024. REUTERS/Rachel Wisniewski

With its 19 electoral votes, Pennsylvania is considered a must-win for both Trump and his Democratic rival, Vice President Kamala Harris, who are virtually neck-and-neck in the state. In 2020, Democrat Joe Biden defeated then-President Trump by about 1 percentage point.

Democratic presidential nominee U.S. Vice President Kamala Harris reacts during a campaign rally in Harrisburg, Pennsylvania, U.S., October 30, 2024. REUTERS/Eloisa Lopez

While Pennsylvania faced an avalanche of unsuccessful litigation by Trump and his allies challenging the 2020 results, those efforts largely emerged after Election Day. Now, the push to cast doubt on the state’s electoral system is already well under way.

Posts suggesting or declaring systemic fraud in Pennsylvania, propelled by Elon Musk and other pro-Trump influencers on Musk’s X platform, are outpacing public officials’ attempts to explain the realities on the ground. Authorities say the posters are misrepresenting routine or unverified problems, leading to a false narrative about electoral malfeasance.

“Pennsylvania is cheating, and getting caught, at large scale levels rarely seen before,” Trump declared to his nearly eight million Truth Social followers Wednesday morning, demanding an immediate law-enforcement intervention.

Foreign operatives also appear to be playing a role in the escalating effort to portray chaos in Pennsylvania ahead of Nov. 5. Recent viral footage purportedly showing a person destroying mail-in ballots was a hoax created by unspecified Russian actors, according to the Office of the Director of National Intelligence and two other federal agencies.

Many Pennsylvania claims are circulating on X, in an “Election Integrity Community” recently launched by Musk’s PAC as a repository for people to share suspected fraud. Claims shared in the group include one from conspiracy theorist Alex Jones of a bus of “non-English speaking ‘citizens’ fast-tracked to the front” of voting lines.

Conservative activist Scott Presler, who heads a voter-registration group called Early Vote Action, is among those zeroed in on Pennsylvania.

On Sunday, Presler thrust Luzerne County into the national spotlight, relaying a “scoop” to his 1.7 million X followers. He shared a video of a county official speaking at a public meeting about a batch of last-minute voter applications, some dating to June, delivered by a former election worker. “If we do not address these inconsistencies,” Presler wrote. “Pennsylvania voters cannot have confidence in fair & free elections.”

“This needs to be investigated,” Musk chimed in, reposting Presler’s message to his own 202 million followers. That had been reposted 17,000 times as of Wednesday.

Luzerne County District Attorney Sam Sanguedolce, a Republican, said Monday his office did investigate. “None of the registrations were fraudulent,” he said in a news release.

Yet Luzerne County Manager Romilda Crocamo doesn’t expect the suspicion to end. “There are people that, no matter what we say or do, are not going to believe that we’re following the system,” she said.

While administrative hiccups or local complaints always arise in elections, the difference now in Pennsylvania is that they are being spread globally by influencers with millions of followers.

In less than 30 minutes Tuesday, the complaint about county workers in Bucks County outside Philadelphia closing down a line too early raced across social media—from the X account of the Trump campaign’s political director, to reposts from conservative celebrities, including commentator Jack Posobiec, to his two million followers.

Bucks officials blamed miscommunication for the fact that people lined up to apply for an on-demand mail ballot were “briefly told they could not be accommodated.”

Trump’s campaign on Wednesday sued the county, claiming its officials violated state election law by turning away voters. Later in the day, a judge extended the deadline for voters to apply for and receive mail-in ballots in person at a local election office.

Trump has called out two Pennsylvania counties, Lancaster and York, for what he said was election fraud.

Supporters react as Republican presidential nominee and former U.S. President Donald Trump attends a campaign event, in Allentown, Pennsylvania, U.S., October 29, 2024. REUTERS/Brendan McDermid

Authorities in Lancaster County, about 70 miles west of Philadelphia, said Friday they were investigating about 2,500 possible fraudulent voter-registration applications submitted to the elections board on the cusp of the registration deadline. The workers noticed irregularities, didn’t process the applications and referred them to law enforcement.

Officials said the potentially fraudulent applications included people registered in both parties.

Schmidt, the Republican chief election official, called Lancaster County’s actions a sign that “safeguards in our voter registration process are working.”

During a campaign rally in Allentown Tuesday night, Trump portrayed the investigation as complete—and voter-registration applications as actual votes cast in Lancaster. “They have cheated. We caught them with 2,600 votes. Now we caught them cold, 2,600 votes.” He echoed the same message over social media.

Write to Scott Calvert at scott.calvert@wsj.com, Alyssa Lukpat at alyssa.lukpat@wsj.com and Cameron McWhirter at Cameron.McWhirter@wsj.com