The Supreme Court declined Sen. Lindsey Graham's request to block a subpoena ordering him to testify before a special grand jury investigating allegations of tampering in the 2020 presidential election by then-President Donald Trump.
Lower courts had already denied requests for a pause by Sen. Graham, R-S.C., who is now scheduled to testify on Nov. 17 in Atlanta.
Graham had argued that the Speech or Debate Clause of the Constitution shields him from being forced to appear before the grand jury.
The Supreme Court noted on Tuesday that Graham does not have to testify about matters related to "legislative activity."
"The lower courts also made clear that Senator Graham may return to the District Court should disputes arise regarding the application of the Speech or Debate Clause immunity to specific questions," the Supreme Court wrote in Tuesday's order. "Accordingly, a stay or injunction is not necessary to safeguard the Senator’s Speech or Debate Clause immunity."
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A spokesperson for Sen. Lindsey Graham said Tuesday that the Supreme Court "confirmed that the Constitution’s Speech or Debate Clause applies here."
"They also affirmed that Senator Graham ‘may return to the District Court’ if the District Attorney tries to ask questions about his constitutionally protected activities," Sen. Graham's spokesperson said.
"The Senator’s legal team intends to engage with the District Attorney’s office on next steps to ensure respect for this constitutional immunity."
Justice Clarence Thomas' temporary stay issued last week on Graham's testimony was vacated by Tuesday's order.
Willis opened her investigation last year into a January 2021 phone call between Trump and Georgia Secretary of State Brad Raffensperger, in which the former president pushed Raffensperger to "find" the votes needed for him to win the state.
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Willis subpoenaed Sen. Graham in July and said she is also looking into a November 2020 phone call between Graham and Raffensperger.
Fox News' Shannon Bream and Bill Mears contributed to this report.