Doll, Nuveen's chief equity strategist, said there are two main reasons why Trump would be worse for the markets than Clinton.
"One is uncertainty. Markets hate uncertainty. I don't think we know what [Trump's] policies are. Maybe by November we will, but we don't at the moment and we don't have a lot of back and forth," he told CNBC's "Fast Money: Halftime."
"The other reason is he's been very clear he's going to start a trade war with China, Mexico and a few other places, and we all know trade wars are not good for economic growth and, therefore, they're not good for markets," Doll said.