Oct 28 (Reuters) - Republicans asked the U.S. Supreme Court on Monday to block a judicial decision from Pennsylvania requiring the counting of provisional ballots cast by voters who make mistakes on their mail-in ballots, potentially affecting thousands of votes in the Nov. 5 presidential election.

The Republican National Committee and Republican Party of Pennsylvania asked the justices to put on hold the Oct. 23 Pennsylvania Supreme Court ruling in favor of two Butler County voters who sought to have their provisional ballots counted after their mail-in ballots were rejected during that state's primary election for lacking secrecy envelopes.

Pennsylvania is one of a handful of closely contested states expected to decide the outcome of the presidential race between Republican former President Donald Trump and Democratic Vice President Kamala Harris.

The Republicans in their filing said that Pennsylvania's top court has undermined the state legislature's authority and changed rules too close to the election.

"This is an egregious usurpation of the General Assembly's constitutional authority to set rules for federal elections," the Republicans wrote.

If the justices are not inclined to pause the state court's ruling in its entirety, the Republicans asked for an order segregating the provisional ballots at issue in the case, which potentially would give the U.S. Supreme Court time to review the legal dispute after the election.

Provisional ballots generally protect voters from being excluded from the voting process if their eligibility to vote is uncertain on Election Day. The vote is counted once officials confirm eligibility.

Unlike Butler County, the majority of Pennsylvania's 67 counties already counted provisional ballots from voters whose mail-in ballots are rejected.

Though the case began with two voters challenging a single county's refusal to count their provisional ballots, Republicans intervened in the case to defend the county's decision, while Democrats intervened on the side of the voters.

Republicans contend that, according to the text of Pennsylvania's election law, if a voter's absentee or mail-in ballot is properly received by 8 p.m. on Election Day but is defective, a provisional ballot may not be counted either.

Democrats said that if a mail-in ballot has a defect and cannot be counted, that person has not yet voted and a provisional ballot must be counted.

A divided Pennsylvania Supreme Court agreed, saying that provisional ballots serve the "dual purpose" of preventing double voting while protecting voter's right to have one vote counted. The state constitution's voting protection does not allow for the "disenfranchisement of voters as punishment for failure to conform to the mail-in voting requirements when voters properly availed themselves of the provisional voting mechanism," the state court said.

Republicans asked the U.S. Supreme Court to apply its 2023 ruling that allows the justices to second-guess state courts in certain cases to ensure they do not "arrogate to themselves the power vested in state legislatures to regulate federal elections."

Republicans also asked the Pennsylvania Supreme Court to pause its own ruling pending the U.S. Supreme Court's review. Secretary of the Commonwealth Al Schmidt, a Republican, opposed that request, noting that the litigation concerning just one county cannot be used to prevent all counties from accepting provisional ballots.

"This court should not countenance such chicanery," lawyers for Schmidt said in a filing.

Pennsylvania is one of the states where Trump and his Republican allies launched litigation during the 2020 election that he lost to Democrat Joe Biden. Pennsylvania Republicans in 2020 unsuccessfully appealed to the Supreme Court a ruling by Pennsylvania's top court ordering officials to count mail-in ballots that were postmarked by Election Day and received up to three days later.

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Reporting by Andrew Chung in New York; Editing by Will Dunham