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HARRISBURG — Setting the stage for a postelection showdown, Republicans who control the Pennsylvania Senate will refuse to seat a Democratic senator whose narrow win in November is being challenged in federal court, even as it has been certified by state officials.

Republican leaders confirmed Sen. Jim Brewster of Allegheny County will not be permitted to take the oath of office Tuesday when the legislature returns to launch a new two-year session.

The top Republican in the Senate, Jake Corman, said that he and his colleagues believe a decision in a legal challenge brought by Brewster’s GOP opponent is necessary before the chamber can act.

“Our goal is to get it right, not get it fast,” Corman said.

At the heart of the dispute is Republican Nicole Ziccarelli’s request to throw out several hundred mail ballots that lacked a handwritten date on the outer ballot envelope, as required by state law. Those ballots gave Brewster the edge he needed to eke out a win.

The issue has already been litigated in the state court system, where Ziccarelli ultimately lost.

“We believe this is an illegal, unlawful attempt not to seat Sen. Brewster,” said Senate Minority Leader Jay Costa (D., Allegheny). He described the maneuver as out of the “Trump playbook” of contesting legitimate and certified election results.

Sen. Vincent Hughes (D., Philadelphia) put it this way: “It’s a robbery.”

Speaking to reporters Monday, Corman stressed that the GOP would not attempt to seat Ziccarelli, and dismissed suggestions by Democratic colleagues that politics was playing a role in the decision.

The state constitution gives lawmakers the power to object to swearing in a member if they believe that person does not meet the qualifications to serve. Those conditions include being of a certain age to hold office and being a resident of a legislative district for a certain period of time.

Brewster, who was first elected to the Senate a decade ago, meets all the above, said his lawyer, Cliff Levine.

“What is going on now is way beyond anything envisioned in the constitution,” Levine said. “The Senate majority is taking it upon itself to decide an election.”

In a challenge currently before the U.S. District Court for the Western District of Pennsylvania, Ziccarelli is seeking to throw out 311 mail ballots that Allegheny County election officials counted even though they did not have a handwritten date on the outer envelope.

In neighboring Westmoreland County, sections of which fall within Brewster’s senatorial district, election officials did not count ballots that were not dated by voters. That, Ziccarelli contends through her lawyer Matt Haverstick, violated her due process and equal protection rights.

The campaign made a similar argument before the state Supreme Court, which late last year cleared the way for the votes to be counted.

Lawyers for Brewster, as well as the Democratic Party, have also argued that a federal court lacks jurisdiction to overturn a state-court decision.

Brewster’s win was certified by state election officials in December. Ziccarelli filed a formal request with the Senate late last week, asking the chamber to reject the state’s certification.

On Monday, Corman said he was not concerned about setting a bad precedent, one that would handcuff the chamber to having to bow to any losing candidates in the future who contested results.

“This was a race that was extremely close, within 100 votes,” Corman said. “This unique set of circumstances dictates that the Senate review it and take very seriously the contest.”

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