The Timeline of Presidential Elections

Highly recommended: The Timeline of Presidential Elections: How Campaigns Do (and Do Not) Matter by Robert Erikson and Christopher Wlezien.

“Erikson and Wlezien have amassed data from close to two thousand
national polls covering every presidential election from 1952 to 2008,
allowing them to see how outcomes take shape over the course of an
election year. Polls from the beginning of the year, they show, have
virtually no predictive power. By mid-April, when the candidates have
been identified and matched in pollsters’ trial heats, preferences have
come into focus — and predicted the winner in eleven of the fifteen
elections. But a similar process of forming favorites takes place in the
last six months, during which voters’ intentions change only gradually,
with particular events– including presidential debates — rarely resulting
in dramatic change.”

Ezra Klein read the book and concludes: “The Romney campaign is in trouble.”