From Bloomberg:

James Wallner, a lecturer at American University and former aide to Republican Senators Pat Toomey, Mike Lee and Jeff Sessions, said that the “kind of personalities that develop in the Senate have not necessarily been conducive in this modern age to run for president.”

Successful recent presidents, he said, benefited from “their ability to be popular leaders, to transcend the nuance and detail of policy issues” while “senators tend to go down into the details.”