U.S. Senate candidate John Fetterman and other Democrats filed a federal lawsuit Monday challenging Pennsylvania's plan not to count undated or wrongly dated ballots that have been mailed in.

Why it matters: The issue of mail-in ballots with missing or incorrect dates has become one of the biggest voting disputes ahead of Election Day in the crucial swing state that may tip the balance of power in Congress, per CNN.

• Fetterman is facing a close race with his Trump-endorsed Republican rival Mehmet Oz to replace retiring Sen. Pat Toomey (R-Pa.).

Driving the news: Pennsylvania's divided Supreme Court last week ruled in favor of a lawsuit filed by the Republican National Committee and other groups, who argued that state law requires voters to correctly write the date on a return envelope when mailing in ballots.

• The court ordered county elections officials to "refrain from counting any absentee and mail-in ballots received for the November 8, 2022 general election that are contained in undated or incorrectly dated outer envelopes."

What they're saying: "The Date Instruction imposes unnecessary hurdles that eligible Pennsylvanians must clear to exercise their most fundamental right," the lawsuit states, filed in the U.S. District Court for the Western District of Pennsylvania.

• "This unnecessary impediment violates the Civil Rights Act and the First and Fourteenth Amendments to the U.S. Constitution."

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Editor's note: This article has been updated with further context.