While the president's claim, based on current law, was wrong, his statement at the first presidential debate got at what his campaign and the national Republican Party have alleged is illegal in the key battleground state -- that "poll watchers" aren't allowed to observe voting where absentee and mail ballots are being cast because those locations don't constitute polling places.
"I'm urging my supporters to go into the polls and watch very carefully because that is what has to happen. I am urging them to do it. As you know, today there was a big problem. In Philadelphia, they went in to watch, they were -- they are called poll watchers. A very safe, very nice thing. They were thrown out. They weren't allowed to watch. You know why? Because bad things happen in Philadelphia. Bad things," Trump said during the final minutes of the debate, hours after he tweeted a similar claim, alleging "corruption" was the reason.
But city officials asserted that no law was broken.
"We're working on a plan now to make sure the polls are safe and secure," Philadelphia Mayor Jim Kenney, a Democrat, told ABC News on Wednesday, noting the plans are "certainly more intense" this election. "The president of the United States has requested supporters to go to polling places. I mean, that's never happened before."
Kenney said that while certified poll watchers on Election Day are legal, "simply going to a polling places and milling around and calling people names -- that's against the law."
"Any extracurricular attempts to keep people from voting are illegal and will be dealt with in such a manner," he said.
Nevada Attorney General Aaron Ford responded to the president's statement on Twitter, saying he "wasn't talking about poll watching" when he urged his supporters to "watch very carefully" at polling places.
"He was talking about voter intimidation," Ford tweeted. "Voter intimidation is illegal in Nevada. Believe me when I say it: You do it, and you will be prosecuted."
Tuesday marked the first day of in-person early voting in Philadelphia, the state's largest city and also the state's largest Democratic stronghold. Following a claim a Trump campaign staffer, Mike Roman, tweeted out just hours prior, the president elevated the false claim to his tens of millions of Twitter followers.
But his tweet, and later statement during the debate, was wrong on two fronts: Poll watcher certifications for early voting have not been issued for anyone, including the Trump campaign; this early in-person voting is only at satellite election offices, which are not polling places, according to Nick Custodio, deputy to the chairwoman of the Philadelphia City Commissioners, Lisa Deeley.
At these satellite locations, Philadelphians can register to vote, request mail ballots, fill them out on site, and submit them to clerks, all in one visit.
Unless voters are there to utilize the services on site, they are not allowed in, Custodio said.
"We don't give someone a poll watcher certificate to … watch somebody fill out their ballot at their kitchen table," Al Schmidt, a Republican and one of the city commissioners who run elections, told the Philadelphia Inquirer.
Officials are also limiting the number of people in the election offices due to the pandemic.
"This is particularly important in the current environment as City buildings and offices remain closed to the public due to COVID-19," he said in the statement.
In a call with ABC News, Custodio stressed that this is "not a partisan thing."
"The same rules apply to everybody, as well as the general public," he said. "Only those who are there to avail themselves of the services of the office are able to enter the area."
In July, the Trump campaign and RNC sued Pennsylvania's top election official, Secretary of State Kathy Boockvar, and all 67 counties statewide, over this issue.
Under state election law, candidates are allowed to appoint two poll watchers and political parties are allowed to appoint three, per precinct, who are permitted inside polling places to observe voting, but also challenge the qualification of voters. Poll watchers must be registered to vote in the county where the precinct they are observing is located.
In the complaint, the Trump campaign and RNC not only allege that the poll watcher residency requirement is unconstitutional, but also that not classifying sites where mail-in and absentee voting takes place as polling sites is as well, and that this "poses a severe threat to the credibility and integrity of, and public confidence in, Pennsylvania's elections."
The Trump campaign lawsuit, which challenged other state elections practices as well, has been halted by a federal judge until Oct. 5, to give time for the state court system to weigh in, and earlier in September, in a lawsuit brought forth by the Pennsylvania Democratic Party, the Pennsylvania Supreme Court did -- ruling that the election observer residency requirement was constitutional, citing the fact that elections are county-run.
That ruling, however, did not address the issue of whether these non-polling precinct voting locations should be considered polling places and allow election observers.
For the past four decades, the RNC had been limited in its ability to engage in what it calls "ballot security" operations, but what critics, like the Democrat Party, have pegged as voter suppression. A consent decree the party signed in 1982 expired in 2018, and the party no longer has to get prior court approval for these operations.
"The lack of this check, therefore, means that nobody is looking in advance to make sure that these operations aren't targeting voters in a discriminatory way or aren't set up in a way that creates a real danger of voter intimidation," Wendy Weiser, vice president for the nonpartisan think tank The Brennan Center, told ABC News Wednesday. "There's a worry that without this check, there isn't an effective mechanism in place to prevent that in advance."
Weiser, and others at the Brennan Center, are lawyers for intervening parties in the Trump campaign's Pennsylvania lawsuit. The reason for getting involved, Weiser said, was primarily over the campaign challenging the use of secure ballot drop boxes, and out of concern over "radical legal claims" that the campaign and the RNC are making, beyond just poll watchers, "would be damaging for voting rights and for election administration writ large" if accepted by the courts.
Ronna McDaniel, chairwoman of the RNC, wrote in an August op-ed in The Washington Post that with the expiration of the consent decree, Republicans and Democrats are again on an even playing field.
"Democrats are deploying an 'army of poll watchers' this November, and for the first time in decades the Republican Party can too," she wrote. "Though Democrats say they want to keep the RNC under this consent decree in perpetuity because Republicans will intimidate voters at polling places, it's clear they want to maintain that court-imposed leg up on Republicans."
McDaniel said that counter to what Democratic critics have said, "Our access is about more people voting, not fewer. That's the opposite of Democrats' claims of 'voter suppression.'"
But Weiser said the concern with poll watchers is when they go beyond observing, and actually "interfere in … or disrupt" the voting process, and the RNC has a history of that.
"The fear is that that is what some are trying to mobilize in places like Pennsylvania," Weiser said.
However, Weiser said voters should panic over this.
"There are a set of procedures in most jurisdictions as to what poll workers and election officials will do to handle, de-escalate and stop these kinds of disruptions when they occur," she said. "There's still a set of authorities, whose job it is to ensure that every eligible voter can vote and to ensure that the polling and voting is accessible."
ABC News' Kendall Karson and Elizabeth Thomas contributed reporting.