ATLANTA — The Georgia prosecutor investigating the conduct of former President Donald Trump and his allies after the 2020 election is subpoenaing U.S. Sen. Lindsey Graham, former New York Mayor Rudy Giuliani, and other members of Trump’s campaign legal team to testify before a special grand jury.

Fulton County District Attorney Fani Willis on Tuesday filed petitions with the judge overseeing the special grand jury as part of her investigation into what she alleges was “a multi-state, coordinated plan by the Trump Campaign to influence the results of the November 2020 election in Georgia and elsewhere.”

The move marks a major escalation in a case that could pose a serious legal challenge to the former president as he weighs another White House run. While the special grand jury has already heard from top state officials, Tuesday’s filings directly target several of Trump’s closest allies and advisers, including Giuliani, who led his campaign’s legal efforts to overturn the election results.

“It means the investigation is obviously becoming more intense because those are trusted advisers, those are inner circle people,” said Robert James, former district attorney in DeKalb County, which neighbors Fulton.

The special grand jury has been investigating whether Trump and others illegally tried to meddle in the 2020 presidential election in Georgia as he desperately tried to cling to power after Democrat Joe Biden’s victory. Trump continues to insist that the election was stolen, despite the fact that numerous federal and local officials, a long list of courts, top former campaign staff and even Trump’s own attorney general have all said there is no evidence of the fraud he alleges.

The investigation is separate from that being conducted by a congressional committee that has been examining the events surrounding the deadly insurrection at the U.S. Capitol on Jan. 6 as well as the Department of Justice’s own sprawling probe. Trump is also facing other legal challenges, including in New York, where he, his namesake son and his daughter Ivanka have agreed to answer questions under oath beginning next week in the New York attorney general’s civil investigation into his business practices.

The escalation comes as Trump has been mulling announcing a third presidential run as soon as this summer as he seeks to deflect attention from the ongoing investigations and lock in support before a long list of other potential candidates, such as Florida Gov. Ron DeSantis, make their own moves.

Willis, who took this unusual step of requesting a special grand jury earlier this year, has confirmed that she and her team are looking into a January 2021 phone call in which Trump pushed Georgia Secretary of State Brad Raffensperger to “find” the votes needed for him to win the state. She has said the team is also looking at a November 2020 phone call between Graham and Raffensperger, the abrupt resignation of the U.S. attorney in Atlanta on Jan. 4, 2021, and comments made during December 2020 Georgia legislative committee hearings on the election.