Rep. Devin Nunes, once sidelined by an ethics inquiry from leading the House Intelligence Committee’s Russia probe, is reasserting the full authority of his position as chairman just as the GOP appears poised to challenge special counsel Robert S. Mueller III’s investigation of possible coordination between the Trump campaign and Russian officials. The California Republican was cleared in December of allegations he improperly disclosed classified information while accusing the Obama administration of exposing the identities of Trump affiliates on surveillance reports. Since clearing his name, Nunes has stepped up his attacks on Mueller’s team and the law enforcement agencies around...