Senate Judiciary Committee Chairman Lindsey Graham (R-S.C.) said he plans to call former Trump campaign adviser George Papadopoulos to testify as part of a "deep dive" into the early stages of the FBI probe into Russia's election interference.

"The committee will be looking at the counterintelligence investigation of the Trump campaign. We will call Papadopoulos and we will find out what happened," Graham said at the start of a hearing Tuesday with FBI Director Christopher Wray.

Graham said that the panel would conduct a "deep dive into 2016 surveillance by the FBI," reiterating plans he has long had to investigate the origins of the Russia probe.

Papadopoulos declined to comment Tuesday when reached by The Hill.

Republicans have long raised questions about the origins of the probe into the Trump campaign's ties to Russia, alleging FBI agents were biased against President Trump in their decisions. They have also raised questions about the role played by the controversial Steele dossier in the FBI's application for a warrant to surveil former Trump campaign adviser Carter Page.

Democrats have accused Republicans of promoting baseless conspiracy theories in an attempt to undermine the investigation, which was eventually taken over by now-former special counsel Robert Mueller.

Mueller's 448-page report confirms that the FBI began investigating potential coordination between the Trump campaign and Russia after Papadopoulos told a foreign diplomat that the Trump campaign "had received indications from the Russian government that it could assist the Campaign through the anonymous release of information damaging to candidate [Hillary] Clinton."

Mueller, who will testify before two House committees on Wednesday, did not uncover sufficient evidence to charge members of the campaign with conspiring with Russia to meddle in then election.

Papadopoulos pleaded guilty to lying to the FBI in connection with Mueller's investigation in 2017. He served a short prison term last year and has become a vocal critic of the FBI and the investigation, claiming it was "entrapped" by Western intelligence.

Graham said he would wait to jump-start his investigation until after Justice Department inspector general concludes his own inquiry into the FBI's actions in applying for the Page warrant.

Attorney General William Barr has also opened a separate investigation into whether intelligence collection on the Trump campaign - which he termed "spying" - was adequately predicated. Democrats have accused the attorney general of promoting a conspiracy theory and undermining the intelligence community.

Wray has previously testified that he had no evidence personally that the FBI engaged in illegal surveillance during the 2016 campaign.

Wray was testifying Tuesday as part of a general oversight hearing focused on the FBI.

Under questioning from Graham, Wray said that Russia has not been deterred by sanctions and other measures and is still engaging in foreign influence campaigns against U.S. elections.

"The Russians are absolutely intent on trying to interfere with our elections through foreign influence," Wray said.

When asked if Russia had not been deterred, the FBI director replied: "My view is, until they stop, they haven't been deterred enough."

Updated 11:49 a.m.