A top Senate Republican defended his investigation into the Obama administration’s Ukraine policy against claims from Democrats that he is aiding a Russian disinformation operation.

Sen. Ron Johnson, the chairman of the Senate Homeland Security Committee, is running joint investigations into the Trump-Russia investigators and their reliance on British ex-spy Christopher Steele’s flawed anti-Trump dossier as well as former Vice President Joe Biden’s involvement with Ukraine and his son Hunter Biden’s lucrative position with Ukrainian energy company Burisma along with Senate Finance Committee Chairman Chuck Grassley of Iowa. The Wisconsin Republican announced a subpoena for FBI Director Christopher Wray for documents related to Crossfire Hurricane and released an 11-page defense of his inquiries on Monday pushing back against assertions by Democratic leaders that he received information from Ukrainians tied to Russia.