The tension between the two levels is caused by the fact that each level has political leaders who want to exert as much power as they can over as many areas of law as they can. Neither set of leaders controls the other and therefore they come into competition with one another. The Arizona immigration case illustrates this. Republican leaders in Arizona wanted to exert control over an area of policy that impacted their state. At the same time, Democrats in the national government wanted to keep power over that area of policy for themselves. Neither side controls the other so both sides compete for power. This causes tension between the two levels.