By Rami Ayyub and Alexandra Ulmer

(Reuters) -The prosecutor for Georgia's biggest county on Thursday requested a special grand jury with subpoena power to aid her investigation into then-President Donald Trump's efforts to influence the U.S. state's 2020 election results.

In a letter to Fulton County's chief judge, first reported by the Atlanta Journal-Constitution, District Attorney Fani Willis wrote that multiple witnesses being probed have refused to cooperate absent a subpoena requiring their testimony.

"Therefore, I am hereby requesting ... that a special purpose grand jury be impaneled for the purpose of investigating the facts and circumstances relating directly or indirectly to possible attempts to disrupt the lawful administration of the 2020 elections in the State of Georgia," Willis wrote.

The investigation by Willis, a Democrat, is the most serious probe facing Trump in Georgia after he was recorded in a phone call pressuring Georgia Secretary of State Brad Raffensperger to overturn the state's election results based on unfounded claims of voter fraud.

Willis specifically mentioned that Raffensperger, whom she described as an "essential witness," had indicated he would only take part in an interview once presented with a subpoena.

In a statement, Trump defended what he called his "perfect" phone call and repeated false allegations of voter fraud in the 2020 election.

In a separate legal woe for the Trump family, the U.S. House of Representatives' panel investigating the deadly January 6, 2021, attack on the U.S. Capitol on Thursday requested an interview with Trump's daughter and former White House aide Ivanka Trump.

And earlier this week, New York state's attorney general accused Trump's family business of repeatedly misrepresenting the value of its assets to obtain financial benefits, citing what it said was significant new evidence of possible fraud.

Trump critics hope that his legal problems may ultimately stymie a potential presidential run in 2024.

"It begins," Democratic Senator Sheldon Whitehouse of Rhode Island tweeted after news of the Georgia request.

'Finding' Votes

In her letter, Willis said a special grand jury, which can subpoena witnesses, was needed because jurors can be impaneled for longer periods and focus exclusively on a single probe.

A spokesperson for the superior courts in Fulton County, which encompasses most of the state capital Atlanta, said there was no immediate timeline for a response to Willis' request.

During the January 2, 2021 call, Trump urged Raffensperger, a fellow Republican, to "find" enough votes to overturn his Georgia loss to Democrat Joe Biden. The transcript quotes Trump telling Raffensperger: "I just want to find 11,780 votes," which is the number Trump needed to win Georgia.

Legal experts have said Trump's phone calls may have violated at least three state election laws: conspiracy to commit election fraud, criminal solicitation to commit election fraud and intentional interference with performance of election duties. The possible felony and misdemeanor violations are punishable by fines or imprisonment.

(Reporting by Rami Ayyub and Alexandra Ulmer; Editing by Bill Berkrot, Jonathan Oatis and Cynthia Osterman.)