Senate Republicans say their investigation of Hillary Clinton’s role in approving a deal to sell U.S. uranium mines to a Russian company hinges in part on the testimony of a secret informant in a bribery and extortion scheme inside the same company. The Senate committee searching for Clinton’s alleged wrongdoing is keeping their witness’s name cloaked. However, William D. Campbell, a lobbyist, confirmed to Reuters he is the informant who will testify and provide documents to Congress about the Obama Administration’s 2010 approval of the sale of Uranium One, a Canadian company with uranium mines in the United States, to...