The justices were divided 4-4 Monday, an outcome that upholds a state Supreme Court ruling that allowed election officials to receive and count ballots until Nov. 6, even if they don't have a clear postmark.
Republicans, including President Donald Trump's campaign, have opposed such an extension, arguing that it violates federal law that sets Election Day as the first Tuesday after the first Monday in November and that such a decision constitutionally belongs to lawmakers, not the courts.
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The state Democratic Party and its allies had sought an extension of Pennsylvania's Election Day deadline to count mailed ballots because Democratic-registered voters are requesting mail ballots at a nearly 3-to-1 ratio over Republicans.
In its Sept. 17 ruling, the divided state Supreme Court said ballots must be postmarked by the time polls close and be received by county election boards at 5 p.m. on Nov. 6, three days after the Nov. 3 election. It also said that ballots lacking a clear postmark could be counted unless there was evidence that they were mailed after the polls closed.
-- The Associated Press contributed to this report.
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