HARRISBURG, Pa. (AP) — A lawsuit filed Thursday is challenging Pennsylvania's court-ordered deadline to count mail-in ballots that are received as many as three days after the Nov. 3 election in the presidential battleground state.

Plaintiffs — including four registered voters from Somerset County and a Republican congressional candidate — are asking a federal judge in Pittsburgh to block the deadline extension from going into effect.

It names Gov. Tom Wolf’s top election official, Kathy Boockvar, a Democrat who has argued in the state's courts that a deadline extension was necessary to prevent late-arriving mail-in votes from being thrown out because of postal delays.

It comes 12 days before the election and three days after the U.S. Supreme Court, divided 4-4, rejected a Republican plea making a slightly different argument than Thursday’s lawsuit.

Both sought to block a state Supreme Court ruling that required county election officials to receive and count mailed-in ballots that arrive up until Nov. 6, even if they don’t have a clear postmark, as long as there is no proof it was mailed after the polls closed.

Thursday’s lawsuit said that the court’s deadline extension and the lack of a postmark requirement “will allow for late and otherwise unlawful ballots to be counted.”

That is unconstitutionally unfair to in-person voters and exceeded the court’s authority by exercising a power that is constitutionally vested in Congress and the Legislature, the lawsuit said.

The Republicans' earlier case before the U.S. Supreme Court did not argue that the deadline extension is unfair to people voting in person. With the plaintiffs seeking expedited consideration, the new argument could theoretically arrive at the U.S. Supreme Court after President Donald Trump's nominee, Amy Coney Barrett, is sworn in, providing a tie-breaking vote before the election.

Most states make Election Day the deadline, but more than 20 states — most of which backed Trump in the 2016 election — have a post-Election Day deadline.

Thursday's lawsuit is just one of several currently being fought in the courts and the Legislature over how Pennsylvania's election is being conducted as the state's 20 electoral votes are hotly contested by Trump and his Democratic challenger, former Vice President Joe Biden.

Already, more than 10 election-related lawsuits have been filed in court this year in Pennsylvania, prompting concerns that the presidential vote count will be heavily litigated and dragged out for weeks.

Republicans have characterized their efforts as intended to ensure election integrity and security, while Democrats have accused Republicans of voter suppression and intimidation.

With about 2.9 million mail-in ballots requested so far, registered Democrats have requested about 1.1 million more mail-in ballots than Republicans, or 1.8 million to 700,000, according to state data.

The 11th-hour fight is happening in the shadow of Trump’s claims at a recent rally near Harrisburg that the only way he can lose Pennsylvania to Biden is if Democrats cheat, a claim he also made in 2016′s election.

At least two other pending lawsuits — one filed by Trump's campaign in state court and one filed by Republican congressional candidates in federal court — are trying to force election boards in Philadelphia and Allegheny County, home to Pittsburgh, to let them send observers into satellite election offices.

A Philadelphia judge two weeks ago rejected the Trump campaign's lawsuit, saying state law does not allow campaign representatives to monitor people registering to vote or filling out mail-in ballots in election offices. Trump's campaign is appealing.

Meanwhile, Boockvar has asked the state Supreme Court to declare that state law does not require or permit counties to reject a mail-in ballot solely over a perceived inconsistency between the signature on the ballot and the voter's signature in their registration file.

Such rejections pose “a grave risk of disenfranchisement" and without any opportunity for a voter to attest that the signature is theirs before the ballot is thrown out, her court filing said.

Republican lawmakers and the Trump campaign have argued that the law is clear that election officials must compare the information on the mail-in ballot envelope, including a voter’s signature, to a voter’s information on file to determine a person’s qualifications to vote.

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Follow Marc Levy on Twitter at www.twitter.com/timelywriter.