For the past 40 years, the U.S. has had one fiscally responsible party and one fiscally irresponsible party. What happens when this changes?
Basic Keynesian economics says that deficit spending gives the economy a temporary boost. Faster growth means more jobs, which means people are happier. That probably leads to better electoral performance for the ruling party. So if Keynesianism is correct -- and the evidence mostly suggests that it is -- then every ruling party has an incentive to run big deficits.