Former FBI director James Comey, whose investigation into Hillary Clinton's emails shook the final days of the 2016 presidential campaign, will testify to Congress later this month about his role in a probe of Russian election interference.

Senate Judiciary Committee chairman Lindsey Graham, an ally of President Donald Trump, announced Thursday that Comey would testify September 30 -- 34 days before November's election -- regarding his role in "Crossfire Hurricane," the FBI counter-intelligence investigation into possible links between Trump's campaign and Russia.

Republicans have accused Comey of a double standard in how he treated Clinton's campaign versus that of Trump.

"The day of reckoning is upon us when it comes to Crossfire Hurricane," Graham said in announcing the hearing in which Comey has agreed to testify "without a subpoena."

"He will be respectfully treated, but asked hard questions," Graham said. 

While Robert Mueller, the special counsel who in 2017 took over the investigation into Russian election meddling, has declined to appear citing a lack of time to prepare, other key figures may testify, Graham said.

Negotiations were ongoing, he said, on getting Andrew McCabe, a former FBI deputy director who served as acting director after Comey was dismissed by Trump in 2017, and former agent Peter Strzok to appear as witnesses.

Trump, backed by his Republican allies, has repeatedly criticized Comey, McCabe and Strzok.

Last week Trump tweeted his misspelled fury at Comey, branding him a "disgraced lier and leaker" for launching a "Russia witch hunt." 

In December a US Justice Department report concluded that political bias did not drive the Russia investigation, rebutting Trump's claims that the Federal Bureau of Investigation illegally spied on his campaign.

Attorney General Bill Barr essentially rejected those findings and launched a new probe, results of which may be released in the coming months.