According to this study, democracy improved GDP per capita by 10%. As everyone knows, the United States does not choose the president through a democratic process—or more precisely, it is democratic voting separately in a handful of states that determine who becomes president. If the candidates had to win a national democratic vote, each person getting equal weight in the voting regardless of state of residence, then we can guess that the American GDP would be nearly $2 trillion higher.
The causation would run through many pathways, including more efficient advertising of policy positions, more efficient delivery of government services, more responsiveness to felt needs of most people, and more confidence in the future of the country.