Senate Judiciary Chairman Lindsey Graham said Sunday that he and other Republicans will begin calling witnesses within weeks for hearings related to Hunter Biden’s work in Ukraine, as well as the FBI’s surveillance of former Trump campaign adviser Carter Page.

“We’re going to get to the bottom of this,” Graham pledged in an interview on Fox News’ “Sunday Morning Futures.”

Graham urged Sen. Jim Risch, R-Idaho, chairman of the Senate Foreign Relations Committee, to call the chief of staff to former Secretary of State John Kerry to testify about concerns he reportedly raised about Burisma Holdings, a Ukrainian energy company where Biden served as a director.

Biden joined the firm in 2014, shortly after his father, then-Vice President Joe Biden, took over as the Obama administration’s main liaison to Ukraine following the overthrow of its pro-Russia president, Viktor Yanukovych.

Republicans have questioned whether Joe Biden, as vice president in 2016, improperly pressured the Ukrainian government to shut down an investigation into Burisma. The elder Biden and Democrats have accused Republicans of using the Burisma issue to distract from impeachment proceedings against President Donald Trump.

Democrats allege that Trump pressured Ukraine’s new president, Volodymyr Zelenskyy, to open investigations into the Bidens in exchange for U.S. military aid.

The Senate is poised this week to vote to acquit Trump on two articles of impeachment related to his actions toward Ukraine.

Graham said he also wants to hear from Kerry’s stepson, Christopher Heinz, who was a business partner of Hunter Biden. State Department emails show that Heinz contacted officials at Foggy Bottom on May 13, 2014, to raise red flags about Hunter Biden’s relationship with Burisma, which has been investigated for corruption.

The South Carolina senator said he also wants to hear from George Kent, a State Department official who testified in the Trump impeachment hearings that he raised concerns about Burisma in 2016.

Graham said Republicans will “eventually get to Hunter Biden” after the first round of witnesses are called. He also said he “can prove beyond any doubt” that Joe Biden’s efforts to root out corruption in Ukraine were undermined by his son’s position on Burisma’s board.

“We’re not going to give him a pass on that,” Graham said.

Graham said his committee expects to call former top FBI officials James Comey and Andrew McCabe and former Justice Department officials Rod Rosenstein and Sally Yates to testify about warrants under the Foreign Intelligence Surveillance Act, or FISA, that they signed in order to wiretap Page.

“When it comes to FISA, I’m going to call Rosenstein, Sally Yates, McCabe, and Comey to find out how a warrant was issued to [surveil] Carter Page on four different occasions without adequate foundation and find out about how the Department of Justice and FBI became so out of bounds when it came to Trump,” Graham said.

The Justice Department inspector general issued a report Dec. 9 that found the FBI submitted false and misleading information about Page to obtain the four warrants. The Justice Department conceded Jan. 7 that two of the warrants were “not valid.”

The inspector general’s report said FBI agents failed to inform the secretive court that issues FISA orders about exculpatory information related to Page.

The report also said that investigators failed to disclose numerous problems with the so-called Steele dossier, which the FBI cited extensively in its FISA applications.

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