President Joe Biden is nominating Julie Su, the current deputy and former California official, as his next labor secretary, replacing the departing incumbent, former Boston Mayor Marty Walsh. Su, a civil rights attorney and former head of California’s labor department, was central to negotiations between labor and freight rail companies late last year, working to avert an economically debilitating strike. If confirmed by the Senate, Su would also be the first Asian American in the Biden administration to serve in the Cabinet at the secretary level.