Super Tuesday was just this week. Soon it will be time for the political conventions and Presidential election. This is a wonderful time to take advantage of current events and teach about U.S. government and elections. Congress for Kids is a helpful website to use for this purpose. Designed for kids 4th grade and up, it has information on the Constitution, the different branches of government, elections, and citizenship.
The information on the site is pretty basic, but makes a great starting point when studying about these topics.