Senate Judiciary Committee Chairman Lindsey Graham (R-S.C.) on Tuesday invited Rudy Giuliani to testify about what he called “disturbing allegations” brought forward by President Donald Trump’s personal attorney regarding “corruption in Ukraine” and the firing of a prosecutor there.

“Given the House of Representatives’ behavior, it is time for the Senate to inquire about corruption and other improprieties involving Ukraine,” Graham wrote in a tweet, inviting Giuliani to appear before his panel.

Giuliani and Trump have repeatedly made the unsubstantiated allegation that former Vice President Joe Biden, who is running for the Democratic presidential nomination to face Trump in 2020, pushed for the firing of Ukrainian prosecutor Viktor Shokin while Biden was in office in order to shield Ukranian energy company Burisma, which his son Hunter Biden served on the board of. However, there is no evidence Biden intentionally tried to help his son by urging Shokin’s ouster, and both Bidens have denied any wrongdoing. Biden and others in the Obama administration have said they urged for Shokin’s dismissal because he was accused of neglecting corruption in his office and for failing to bring corruption cases. The calls for reforms to Ukraine’s prosecutor general’s office at the time also came from other nations, as well as a pair of Republican senators ― Rob Portman of Ohio and Ron Johnson of Wisconsin. Giuliani has found himself ensnared in the House impeachment probe, which was fueled by Trump’s repeated requests for Ukrainian President Volodymyr Zelensky to aid both Giuliani and Attorney General William Barr in an investigation of Biden. Democrats on the House Intelligence Committee last week subpoenaed Giuliani for documents dating back to January 2017 regarding his communications with Ukrainian officials.