Republican senator seeks subpoena in probe into Bidens, Ukraine Republican Senator Ron Johnson, the chair of the Senate Homeland Security Committee, is pushing for a subpoena of a former Ukrainian diplomat tied to the investigation into the Ukrainian natural gas company Burisma, where former Vice President Joe Biden's son sat on the board. CBS News senior investigative correspondent Catherine Herridge spoke to CBSN about why the calls for testimony are coming just days after Biden surged in the race for the Democratic presidential nomination.