Given the overwhelming tendency of incumbents to win elections, what are three possible reasons why an incumbent loses?
There are many possible reasons why an incumbent might lose an election. One is that their constituents might be actually dissatisfied with their performance. This is especially true of executives: it is no secret that presidents and governors often take the blame for a bad economy, but it can happen to legislators as well. Another (related) reason is that incumbents have a record to attack. It is not that difficult to locate some unpopular measure that an incumbent voted for, whereas those with less of a record, despite their lack of experience, also have less to criticize. Finally, incumbents could suffer from dissatisfaction with their party, especially when they are in the same party as an unpopular executive. Anti-Bush and anti-Obama sentiment played a role in bringing out Democratic voters in 2006 and 2010, respectively.