WASHINGTON—President Joe Biden plans to nominate the No. 2 auto safety official to head the National Highway Traffic Safety Administration (NHTSA), the White House confirmed on Tuesday. Steven Cliff, who has been deputy administrator since February, has been a key figure in the Biden administration’s proposed rewrite of fuel economy standards through 2026. He is also overseeing the department’s safety probe of Tesla Inc. and investigation of whether 30 million vehicles produced by nearly two dozen automakers have unsafe air bags. Reuters first reported Cliff’s planned nomination earlier on Tuesday. NHTSA later confirmed a Reuters report that Duke University engineering and computer science professor Missy Cummings is being named a new senior adviser for safety at NHTSA, saying in a statement it looks forward “to leveraging her experience and leadership in safety and autonomous technologies.” The agency added Cummings was being named for a “temporary assignment” under the Intergovernmental Personnel …