Why do U.S. senators have a harder time winning reelection than do members of the House of Representatives? Jonathan S. Krasno argues that Senate elections are more competitive because Senate challengers are more likely to be experienced politicians who wage intense, visible, and costly campaigns than are those who take on House incumbents.

Krasno, the first scholar to draw on the Senate Study--a survey conducted by the National Election Study specifically to analyze Senate elections--clearly and compellingly disposes of previous explanations for the vulnerability of senators. He argues...