Votes in federal elections are cast and counted in a highly decentralized and variable fashion, with no uniform ballots and few national standards.
The increase in straight-ticket party voting in recent years means that competitive congressional races can tip one way or the other depending on the showing of the candidates at the top of the ticket.
In addition to the decline in competition, American politics today is characterized by a growing ideological polarization between the two major political parties.
Presidents are elected not by direct popular vote but by 538 members of the Electoral College.
Responsibility for overseeing the implementation of election law typically resides with partisan officials, many with public stakes in the election outcome.
Partisanship particularly increased after the 1994 elections and then the appearance of the first unified Republican government since the 1950s.
Incumbency adds a layer of advantage on top of this party dominance. But rather than foster an environment in which members of Congress feel free to buck popular sentiment and wrestle seriously with the problems confronting the country, it reinforces the ideological divide between the parties.
While Republican voters have remained universally supportive of their President, Democrats and Independents are returning to a more naturally critical stance.
Democrats do best in urban centers, Republicans in outer suburbs and rural areas.
Further-more, partisan attachments powerfully shape political perceptions, beliefs and values, and incumbents enjoy advantages well beyond the way in which their districts are configured.