Republican officials and candidates in at least three battleground states are pushing to disqualify thousands of mail ballots after urging their own supporters to vote on Election Day, in what critics are calling a concerted attempt at partisan voter suppression.

In Pennsylvania, the state Supreme Court has agreed with the Republican National Committee that election officials should not count ballots on which the voter neglected to put a date on the outer envelope – even in cases when the ballots arrive before Election Day. Thousands of ballots have been set aside as a result, enough to swing a close race.

In Michigan, Kristina Karamo, the Republican nominee for secretary of state, sued the top election official in Detroit last month, seeking to toss absentee ballots not cast in person with an ID, even though that runs contrary to state requirements. When asked in a recent court hearing, Karamo’s lawyer declined to say why the suit targets Detroit, a heavily Democratic, majority-Black city, and not the entire state.

And in Wisconsin, Republicans won a court ruling that will prevent some mail ballots from being counted when the required witness address is not complete.

Over the past two years, Republicans have waged a sustained campaign against alleged voter fraud. Experts say the litigation – which could significantly affect Tuesday’s vote – represents a parallel strategy of suing to disqualify mail ballots based on technicalities. While the rejections may have some basis in state law, experts say they appear to go against a principle, enshrined in federal law, of not disenfranchising voters for minor errors.

The suits coincide with a systematic attempt by Republicans – led by former president Donald Trump – to persuade GOP voters to cast their ballots only on Election Day. Critics argue that the overall purpose is to separate Republicans and Democrats by method of voting and then to use lawsuits to void mail ballots that are disproportionately Democratic.

“They’re looking for every advantage they can get, and they’ve calculated that this is a way that they can win more seats,” said Sylvia Albert, director of voting and elections for Common Cause, a nonpartisan democracy advocacy organization. “Research has shown that absentee ballots are more likely to be discarded if they are voted by young people and people of color, which are not generally seen as the Republican base.”

Albert said legal battles over mail ballot eligibility have the potential to delay results and even change outcomes. In some cases, the disputes could wind up before the U.S. Supreme Court.

The potential for chaos is especially high in Pennsylvania, where the legal fight is ongoing and could influence or postpone the outcome in some of the state’s tightest races, including a contest that could determine control of the U.S. Senate.

Republican National Committee spokeswoman Emma Vaughn said in a statement that the committee sued in Pennsylvania “because we are simply asking for counties to follow the state law, which by the way, dozens of Democrats supported.”

“We look forward to continuing our legal actions to ensure that elections are administered in accordance with this bipartisan rule of law,” Vaughn added.

Pennsylvania Gov. Tom Wolf-D, issued a statement Sunday night in which he asserted that “no voter should be disenfranchised simply because they made a minor error in filling out their ballot.”

“This was not a controversial concept in our country or our commonwealth until recently, with the rise of the Big Lie and the efforts to spread mis- and disinformation in the days leading up to the general election,” Wolf continued. “I urge counties to continue to ensure that every vote counts.”

Voting rights groups and others have launched a sprawling volunteer-run effort to notify voters across Pennsylvania whose ballots had been rejected and needed to be fixed or replaced. At least 7,000 such ballots have been rejected statewide for a variety of reasons, including the missing date, according to data compiled by the Pennsylvania Department of State. Activists said the figure is probably much higher because many counties have refused to publish the information.

People wait in line to resubmit their ballots and vote early at City Hall in Philadelphia on Monday. Caroline Gutman/The Washington Post

Nowhere has the effort been more intense than in Philadelphia, the state’s largest city and a Democratic bastion, where more than 2,000 such ballots have been rejected. Election officials posted lists of voters online with instructions to come to City Hall up through Election Day to cast a replacement ballot, and on Monday, they sent a robocall out to voters whose numbers they had.

By afternoon, a two-hour line to cast a replacement vote at City Hall snaked outside and into the building’s courtyard as volunteers supplied pizza and bottled water.

“I’m lucky. I could wait in line and do this,” said Melissa Sherwood, a 25-year-old Democrat who works from home. “Some people who don’t have that luxury probably took one look at the line and said, ‘No way.’ ”

While some voters said they had been contacted about their ballots multiple times, others said they hadn’t heard anything until they received a call from a Washington Post reporter.

“Our fear is there will probably be several thousand Philadelphians who lawfully attempted to vote and their votes will not count,” said Benjamin Abella, an emergency physician volunteering with a group of fellow doctors to notify voters that they need to fix their ballots.

Even when notified, not everyone can make it to City Hall.

“I am totally disabled,” said Jean Terrizzi, 95, who was listed as having returned a ballot with a missing date. She added that she had an important medical appointment on Monday and would just have to “let it go” and not have her vote counted.

“This voting situation is terrible,” she said, declining to state her political affiliation. “It’s very disgraceful.”

Election officials are also braced for a repeat of a protracted standoff following Pennsylvania’s May primary between state officials and three counties – Berks, Fayette and Lancaster – that refused to include undated ballots in their certified results.

Wolf’s administration sued those counties in July to force them to include the ballots, the majority of which were cast by Democrats, court records show. In August, a state judge ordered the counties to include “all lawfully cast ballots,” including those with missing dates, in their certified results.

Republicans then successfully persuaded the state Supreme Court to reverse that policy for the general election in a decision released last week. The state court deadlocked on whether rejecting the ballots was a violation of voters’ federal civil rights.

Common Cause and others quickly filed a federal suit seeking to overturn the state court ruling on the grounds that rejecting ballots over a technical error violates the Civil Rights Act.

The date printed on the envelope of a mail ballot is a “meaningless technicality” that has no bearing on officials’ ability to judge whether the ballot has been cast on time by a qualified voter, the complaint says. The case is pending.

The date requirement originated when Pennsylvania allowed certain ballots to arrive after Election Day so long as the voter attested to casting it by the date of the election. Now, ballots must be received by Election Day.

The federal courts have already weighed in on the issue: This year, the U.S. Court of Appeals for the 3rd Circuit found that not counting undated mail ballots is a violation of federal civil rights law. However, the U.S. Supreme Court injected uncertainty into the issue by vacating that decision and instructing that the case be dismissed as moot because the election in question had already passed.

Republicans filed a separate lawsuit this year to block counties from notifying voters who neglected to date their ballots to give them the chance to fix them. The effort failed, but counties may choose whether to do so, meaning not all voters will be given the opportunity.

Small numbers of votes could make a difference in the sort of close races to which Pennsylvania has become accustomed.

“If you can eliminate 1 percent of the votes and they tend to lean Democratic, then that gives you that statistical advantage,” said Clifford Levine, a Pittsburgh-based election lawyer for Democrats.

“This is not about stopping fraud,” Levine said. “It’s about discounting mail ballots. There’s just no question.”

Republican candidates in Pennsylvania, including gubernatorial nominee Doug Mastriano, have been vocal in urging supporters to cast ballots on Election Day, not by mail.

Jeff Mandell, a Democratic election lawyer in Wisconsin, said there has been less of a coordinated effort in that state to steer Republicans toward Election Day, although Trump made that pitch at an appearance this year.

Under Wisconsin law, an absentee voter must find a witness – usually a spouse, relative or friend – to attest that the voter legally completed the ballot. The witness must sign the ballot envelope and provide an address.

Republicans successfully sued this year to toss guidance from the Wisconsin Elections Commission allowing local election officials to fill in incomplete witness addresses on ballots.

“There is a concerted effort by the Republican infrastructure, the party, and others working with it, as well as Republican leaders in the legislature, to undermine absentee voting and make it harder for people to vote that way,” Mandell said.

Wisconsin Republicans who spoke out in favor of the suit said state law is clear that only a voter may correct an incomplete address.

“Lawless ballot curing cannot and will not be allowed to continue,” Republican Senate Majority Leader Devin LeMahieu said in a statement issued at the time.

A Michigan judge on Monday dismissed the lawsuit by Karamo, the GOP secretary of state nominee, in a strongly worded opinion.

“Plaintiffs’ failure to produce any evidence that the procedures for this Nov. 8 election violate state or federal election law demonizes the Detroit City Clerk, her office staff, and the 1,200 volunteers working this election,” wrote state Circuit Court Judge Timothy M. Kenny.

Even though the suit was dismissed, other challenges are playing out: In recent days, county clerks across Michigan have received emails from organized groups attempting to dispute the eligibility of voters who requested or cast absentee ballots, suggesting there could be more litigation to come.

The Washington Post’s Tom Hamburger in Washington and Patrick Marley in Madison, Wis., contributed to this report.