For years, UPS truck drivers asked the delivery giant to install air-conditioning in its ubiquitous brown vans. The company resisted, even as temperatures climbed and drivers suffered from heatstroke.

Now, David Keeling, a former health and safety executive at UPS who some workers blame for the inaction, is President Trump’s pick to lead the Occupational Safety and Health Administration, the federal agency that regulates workplace safety. A Senate committee is scheduled to vote Thursday on his confirmation.

Mr. Keeling, who spent nearly four decades at UPS before moving to Amazon in 2021, would be taking helm at the agency just as it considers the first federal rule designed to protect as many as 36 million workers from extreme heat. Among other things it would require employers in industries like agriculture, construction and manufacturing to provide water and rest breaks when temperatures pass certain levels.

Labor unions are split on his appointment. The International Brotherhood of Teamsters, which endorsed President Trump, has also backed Mr. Keeling, saying that as a former tradesman with deep experience in health and safety, he was just the person for the federal government’s top worker-safety job.

“He’s someone we feel we are able to have a conversation with,” said Kara Deniz, a spokeswoman for the Teamsters, which represents 340,00 UPS drivers and package handlers.

But some labor advocates, as well as drivers who worked under Mr. Keeling at UPS, say they struggled to get the company to take measures to address dangerous heat conditions. That included requests for air-conditioning in delivery trucks and personal cooling gear. They say UPS told them that air-conditioning wouldn’t be effective because of the trucks’ frequent stops.