The word "sophomore" is commonly used to refer to someone in their second year of high school or college. In this case, it refers to someone in their second term of Congress. It originated from sophumer, which came from sophum ("sophism"), which probably came from the Middle English sophime ("sophism"). It is an alteration of the Greek sophos ("wise") + mōros ("foolish").
This phenomenon first started in the 1960s. As of 1998, freshman candidates running for a second term now get eight to ten percent more votes than when they were elected for their first term. (Over ninety percent of all incumbent House members are reelected.) Senate members also currently benefit from a sophomore surge, though it is to a lesser degree.
The reason for the sophomore surge is attributed to the fact that congressmen have figured out how to run personal campaigns rather than party campaigns. They make use of their free, or “franked,” mail; frequent home trips; radio and television broadcasts; and service distribution to their districts in order to create a good opinion of themselves, not their party, among their constituents. They also promise to “clean things up” in the federal government if they are re-elected.